If you know D-Jet, that's awesome.
The rust points you mention are common. Check out @jamesinc's build threads for his 240s, Momo and Amy under "Rides" - he's done a lot of rust repairs in the same areas.
240 75-77 and 140 74+ doors will fit straight on. 240 78+ won't - different locking mechanism.
Windscreen surround panel and top scuttle panel rust are commonly caused by the the ridiculous metal clips (rust clips, I call 'em) which hold the windscreen and the mud-collectors (aka windscreen surround brightwork moulds) in place. A 91+ 240 windscreen with the single piece neoprene seal is my preferred solution, but some prefer keeping the mud collectors, rust clips (which are now hard to obtain) and the butyl mess to hold in the screen. If you replace the screen with the better later version, you'll need to plug up the holes in the surround in which the rust clips used to sit.
The late model 240 fuel tanks will fit into a 74, but they have a lift pump and swirl pot inside. The later fuel tanks were retrofitted to 74 models in the USA as a service option because of vapor lock issues. See PDF.
Note also the 74 model fuel tanks have an in-tank filter - that's what the huge brass plug with the square socket-like hole is for. It will most likely not have been looked at anytime this century and my bet is it's falling apart. VP Autoparts stock replacements if you want one. The later 240 tanks don't have this plug or filter.