First numbers are in. 13.5mpg or 17.4l/100km. Could be worse I suppose.
Looks like a modern drive train is going to win out. Unfortunately flatties leak badly, and there seems to be no real fix. Right now Ive leaks from just about anywhere possible, including the diff and trans. Also late flatheads lost their hardened valve seats
Looking at a 1UZ, T5 and Ford 8.8. Modern leak free and probably about 200lbs lighter than stock.
There's a 4x8 sheet of checker plate in the back. Enough for a good bit of undertray. Though it's probably a bit on the heavy side.