The FE changed to a three bolt motor mount about 1965. The older ones were two bolt. At least I think I am right. The engine size should not matter, just the year of the block.
all fe blocks will fit when the changed mount locations in 65 they left the old holes in place.however if your going the other way putting a 64 or earlier block in a later car your out of luck.they are still close and it doesnt take much to make them fit.