Hmmm, looks like a pretty straightforward restoration to me, I've restored plenty a lot worse. Why the caginess as to the chassis number, is this some 'bitsa'? It certainly does not look like one. You allude to the 'transmission' being later than 1960, how do you know this? You can find out
exactly when the axle and gear box were made by checking the numbers! Salisbury axles are stamped on the underside of the casing nose with a code. Thus 'A-60-020' would be axle number 20 made in January (month A, Feb month B and so on) 1960. Moss gearbox numbers are all listed and catalogued. I have Norton Gabb's (he built most of them) full list, as has the Morgan Registry, ask them. If you know the chassis number (which you purport to) and all the numbers you can very easily find out everything about your car and any others of which it is comprised even down to the colour of Chalie Curtiss's underpants when he tested it, which he surely will have! I wouldn't worry about the engine rebuild, that's the easy part!

Most Plus Fours have had a transplant or two in their lives. I expect the original engine is bean-cans by now, but you can easily replicate it. PM me is you want some further info, but I will need a bit more from you also.
When you have finished that, try this!!
Ripe for Restoration! Colin M.