What will fit / work?

You will actually have to use the original 273 motor mounts and modify them for the 340/360. Not a problem as it's just a small weld job.

You will also have to address the timing cover/water pump issue and use the earlier cast iron water pump and pulley setup if you're using an original V8 radiator.

And, pertaining to the car, if it's a slant 6 car, you will need the V8 cable style gas pedal assembly, the 273 oil pan, and a V8 centerlink! And of course, the aforementioned exhaust issue! All easily overcome with parts and knowledge from your friends at FABO. Welcome to the wonderful world of early A bodies!!