not any changes in the engine itself that meant much of anything, most changes were add on/ bolt on in those years. If you put a 68 engine in there or you put a 74 engine in there nobody at the local drive in would know the difference....as long as you painted it the right color for your year.... and there were no compression differences etc those years. if it were me Id be shaving the head and possibly changing the cam, no matter what year engine I found at the very least... things that cant be "seen".