Finding 360 engine/tranny to replace slant six

So, if I wanted to do a pre-EFI engine does anyone know what years I need to look for? Also, at what point did the tranny change so much that it requires tunnel modifications? Are the transmissions different on different types of vehicles for the same year (i.e. car vs. truck vs. RV)?

Sorry if my questions seem stupid, I really have tried to research it as best I could. I'm just trying to plan out the least painful approach to the swap.