Are diesel bodies swappable between Lionel GP MPC diesels and more modern Lionel or MTH?

You may be correct up til the mid 1990's. After the metal frame for traditional diesels, Lionel went with a generic plastic base for GP and ALCo RS diesels and the shells screw from the bottom. 

I've swapped some MPC shells onto Williams frames, screw through the ends mounting was the same, but because of the True-Blast horn I don't think that's what your looking for.  As stated above many of the newer diesels have 4 screws up through the frame into bolsters on the corners so this wouldn't work unless you want to create bolsters (not easy but I've seen posts on repairs that may work).

I'm thinking F3's might work. Just because they fit, doesn't mean they'll look good. A lot of engines have painted frames and handrails.

Thanks (I guess) , guys. Another good idea down the drain. I don't have any F3 units I want to upgrade,