slant 6 question

I haven't been on here in several years due to building new house and garage. Hadnt had the time. Also the cars I had then were not slant 6 cars but I have picked up 3 more a body cars and now I am dealing with slant 6 issues. Question is are most of these slant 6 /904 interchangeable. I read somewhere about the 1966 being a stand alone year that the crank is different and it has to mate with a 1966 only. Also can you put a slant six from 1967 Dart down in a 1976 Dart Sport with no real problems. I believe I have either a cracked head or bad head gasket on the 76 Dart Sport and for now I plan on dropping a different motor in ( from a 67 Dart )and going through that engine. just wanted to know a little bit before I get into it.