Restoring a Top Hat

[Zetomagneto]

There are quite a few cars running around on shortened LWB chassis frames, that's eminently do-able I believe.
The only care point I would suggest is to appropriately manage the transfer (or otherwise) of the chassis number to avoid any future paperwork issues - probably better thought about sooner rather than later.

Indeed! Although the PO couldn't find the original chassis number years ago and the car has a V5 issued on a made up "VIN". I'm not going to rock that particular boat......

Anyhow, only 40% of this chassis will have been replaced and the original chassis number is actually stamped on the rear crossmember (I'm retaining that bit), along with the engine and body number. Just need to make sure the number is visible on the front N/S chassis rail.....

The only other point is that the later rails are about 1/8" deeper at the rear where the springs fit. I'll cross that bridge when I get to it!

I would be trying a bit harder to retain the original rails, this Chassis is going to end up a right mishmash of parts.There are numerous points wrong with the replacement rails you have and if DVLA become involved this could turn out a right mess.
Repair and use all the original Chassis, it keeps the car original and no need to involve DVLA.
Will also make the car more valuable / saleable if you decide at some time to sell it.
I am sure that in the right hands the rails could be repaired . I know you have spent time and money so far but I am sure you could achieve a better result, a car of this rarity deserves it.
I do not know this company, but have a look, it will give you an idea of what can be done : http://www.chassisalignment.co.uk/