I would recommend only lifting the wood frame from the chassis after completing as many repairs as possible. The chassis gives a firm foundation to the wood frame during repairs, to help retain the original dimensions. It also means that the wood frame won't need reinforcing once you do lift it.
Before going into the mundane details, there is the important issue of the seal to consider.
Damp proof seal
The original damp-proof course seal between the wood frame and the chassis, was made from a cloth (probably asbestos) soaked in bitumen. The bitumen made a water-tight seal to protect the wood and steel surfaces, and also prevent moisture in the wood from attacking the chassis. The soft cloth allowed for any unevenness of the chassis to be accommodated.
Alan Alderwick's book on wood frame restoring, recommended using modern damp-proof course and silicone-rubber sealant. Alternatively, he suggested 3mm thick polyethylene strips. He also claimed that bitumen loses its sealing properties after drying out. This raises a number of issues as follows:
1) Modern damp-proof course is made from polyethylene to which hardly anything will stick (except specialist adhesives and sealants).
2) Modern damp-proof course is only 0.5mm thick. My AC's original seal was folded over double, to make approx. 4mm thickness which then compressed to 3mm under load.
3) The seal needs to be sufficiently thick and soft to seal the whole surface.
4) My seals were still stuck to about 95% of the surfaces after 65 years, so the bitumen had worked well.
5) Polyethylene is not fire resistant. Some types are classed as "fire retardant", but that means it still burns in a fire, but extinguishes after removing the flame.
6) Many silicone-rubber sealants release acid as they cure, so you need to get a neutral curing sealant.
7) Silicone-rubber sealant absorbs surrounding moisture as it cures, and thus there is a limit to the depth it can cure. That is, it might not fully cure if most of it is not in contact with the surrounding air.
8) I found that the chassis outriggers had distorted immediately around each bolt hole for the wood frame. Under each nut was a spring washer, which would have concentrated a lot of force on a small area of chassis plate. But this buckling is probably a good thing: It allows close contact between wood and metal around each bolt, for secure joints, and prevents most of the new seal from compressing too much if it is soft.
9) The correct final compressed thickness of the seals is important to maintain good alignment between main bodyshell/doors and the front wings.
I decided to try 5mm thick closed-cell neoprene foam without any added sealant (silicone-rubber sealant doesn't usually stick to neoprene). Closed-cell foam is water-proof. Neoprene has some degree of fire resistance (rather than just "retardance") and has a reputation for resisting weathering. With hindsight, I would have preferred 6mm thick (or 1/4"). When the bolts were tightened, on my AC, its thickness varied between 2mm and 2.5mm, with greater thickness at certain points. A useful test is to see how well the bolts go into the sides of the chassis through the boot side panels. My right-hand ones fitted perfectly, but the left-hand ones were tight due to the wood frame being a small fraction lower than before.
I did establish that the neoprone foam sticks well to the painted surface of the wood frame. I noticed this when I lifted the frame after test fitting, so I am confident that it makes a water-tight seal.
Note that the rear of the wood frame is bolted directly to the chassis without a seal. That is, the rear corners onto the chassis brackets, and also the boot side panels against the chassis side-rails. For these, I have applied a good quality silicone-rubber sealant: Dow Corning 791. This is advertised as allowing for plenty of joint movement in service, and is also neutral curing.
Please note: My use of neoprene closed-cell foam, is not tried and tested (to my knowledge), and I recommend that you consider what is the best material and method for your car.
The above photos show the wood frame lifted a few inches, revealing the top of the chassis. On the left is the original seal, and on the right, the new one.
Nuts and bolts
The wood frame is bolted down partly with coachbolts. These have Whitworth threads and square nuts, along with spring washers. I managed to obtain some new-old stock of GKN coachbolts with their square nuts. If you are not bothered about this level of originality, then there is nothing wrong with substituting UNC or metric coarse. As well as the spring washers, I added flat washers to spread the load under the (already buckled) chassis plates. There are also several high-tensile BSF hex bolts, and I fitted zinc-plated items. Coat all of these bolts with plenty of clear Waxoyl for rust prevention, without damage to the wood.
When I removed the original bolts from my frame, most of the coachbolts were bent, and in a direction consistent with the wood frame having shifted fowards and slightly to the right. Maybe this was caused by a frontal crash? The sloping sections of the chassis tend to limit any scope for forward movement.
Any bolts passing through wood are likely to need retightening after a period of time. The wood compresses (or the edges of bolts/washers cut the wood fibres).
I would recommend making some alignment jigs to ensure that the wood frame goes back onto the chassis in exactly the same position. With so many bolts, it should end up very closely, if not perfectly, aligned, but it is best to be very sure. I cobbled together jigs from a few scraps of wood, so that they would fit against the sillboards and the chassis crusiform. Also another pair of jigs for the rear wooden crossmember, to align with the rear chassis brackets.
Some of the nuts and bolts are sheltered from the elements and may unscrew easily. Others may be seized and the bolts spin in their holes. You might be able to grip the bolt head in locking pliers, or cut a slot in the head for a large screw-driver.
The next task is to break the seal between the sealing strips and the two mating surfaces. A wide chisel tapped between the folded layers of seal, will help. Otherwise it is quite straight forward. Lift the frame, and it should ease off, being careful not to twist it. The wood frame was even lighter than I expected, being very easy to lift one end single-handed.
If you are fortunate, you will have enough garage space to place the wood frame alongside the chassis. Two people should be able to lift it, being careful not to catch the wood against the chassis outriggers. If like me, you are working in a single garage, then the wood frame needs to be lifted a few inches. I used three jacks. One near each front corner, and one in the centre of the rear cross-member (but with a plank of wood to spread the bending load). Using three jacks will avoid twisting the frame, but one has to be careful that it does not tilt or slide sideways. Then I rested mine on four axle-stands (under the inner sills), but with the rear jack in place to take the weight of the rear end.
Repairs and painting
Now you have easy access to complete repairs under the wood frame and to paint all those areas now exposed (chassis and wood frame). On one of my main sillboards, there were some splits at its rear end. Apart from pushing glue into the splits, I reinforced the area with a small patch of ash with its grain going across the car. You will also have access to the tubular cross-member of the chassis, just forward of the rear axle. Debris accumulates in this where it dips under the prop-shaft. I attached a cloth to a length of old TV aerial lead, and pulled it through the tube.
Industrial suppliers charge a lot for materials, but I found an ebay seller with strips of neoprene closed-cell foam in a choice of widths. I purchased 45mm wide, 5mm thick, strip with self-adhesive on one side. I also bought an additional small sheet for cutting out pieces for the outriggers. With the top of the chassis completely free of grease and dirt, it's an easy task to stick down this material. Then poke out the bolt holes. As I mentioned above, seals slightly thicker than 5mm might be better?
These brackets bolt to the tops of the rear jacking points, with a small gap between the brackets and the wood frame. On my AC, this gap was filled with a scrap of aluminium sheet folded in two, and stuck in with bitumen. As an alternative approach, I fitted some off-cuts of my neoprene seals to prevent any dirt getting trapped in the gap.
With fast setting sealant to apply, and 28 bolts to go in, it is best to do a dry run first. Access under the car to fit nuts and washers, is quite restricted. Although one shouldn't use sockets on square nuts, I did use a 16mm socket for the initial tightening, to speed up the process. The bolts next to the hinge pillars are hard to access from underneath, so I balanced the nut and washers on a socket (these have hex nuts). Check that the bolts for the boot side panels, will still go into their holes (to show that the frame is at the correct height). Once happy with the test fitting, then sealant should be applied to the rear mounting brackets. I did not apply sealant for the boot side panels at this stage, because the bolts will be removed at a later date for inner wing installation. However, you might want to restore/make and fit the inner rear wings while the body is off the chassis? It is quite awkward to install the inner wings when the brake backplates are in the way. I suspect that the later type of inner wing (which has a smaller axle cut-out, from chassis EL1307 onwards) may be harder still to install, unless the body is off the chassis (or the rear axle removed). But then again, it may be a good idea to modify those inner wings by removing the channel edges that fold over the edge of the plywood boot sides? Those channels are serious traps for water and dirt, and it might be better to simply fit screws to secure the inner wings? With those channels removed, inner wing installation becomes a little easier.
With sealant applied to those rear brackets, you can lower the frame down, and man-handle it to align the bolt holes. With the bolts dropped in, check with the alignment jigs and get the position perfect. Then follows the long task of getting all the bolts tightened down. Start with the front two, which are angled forwards and should get the frame in its correct fore/aft position. Recheck the jigs. The rear-most bolts on the sillboards are over another sloping join, but the bolts are actually vertical, so they will tend to pull the frame rearwards. Tighten the small coachbolts on the rear cross-member before the sealant starts to set. Then tighten all the other bolts.
It's a good idea to test fit the doors and front wings to be absolutely sure that everything is still as it should be. And don't forget to check the tightness of the wood frame's mounting bolts at a later date, as it is annoying when one's body falls off!