henryd,
What is meant by "jacking the chassis until its perfectly level"? As you know, these frames are not very torsionally rigid. Nor, frankly,are they all that strong in fore/aft bend.
When we take a car apart here in the shop, we often weld temporary bracing across the cockpit before removing the body. This is to retain original fit. It's sort of the inverse of what I'd advise here.
Don't force the chassis to do anything it doesn't want to do. Of course, you should measure diagonals and all for damage, but beyond that, all you'll do is force the chassis out of its "set". Imagine what would then happen after you went to great pains to get all those trim pieces and door gaps and latches working nicely, and then began using the car. Believe me, it'll "remember" that old set and start heading there like a trail horse back to the feeding shed.
Instead, finish the chassis and suspension. If possible, install the engine and gearbox. If not, simulate its weight and placement. Now you can begin to build a car that won't suddenly start chipping its paint at the corners and such. It also will feel more solid (solid being a relative term here!) and "all of a piece" Fewer bits will vibrate and loosen up.
This advice is not nearly as important with a later car, such as a TD or an A simply because these cars both dave FAR more robust chassis. On these cars, it's the body we brace, but in general, the concept of "freezing" things dimensionally is always worthy of consideration BEFORE the fact
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon