I don't know much firsthand about the RX-7 but I would be inclined to think that the parts might be relatively hard to find, esp aftermarket. Out of what you asked about I would say 5.0 Fox, personally I would take the GM over the Ford but I don't like the 5th gens, I would look for a 5sp 4th gen camaro and drop a 350 in it, easy to do and I would say the 350 has got to be THE most popular motor in the V8 market if not it definetly has more aftermarket support than anything else I can think of.