I seem to remember that once the 1972 pancake engine came out the muffler 
connections were all the same, up to 1979, ie.4 bolts per side, two tubes 
per side. I remember various fittings for pollution control EGR that existed 
on some years but which could be 'blanked' off with a bit of flat metal. I'm 
not sure about the heat exchangers being as widely swappable, is your 
supplier willing to let you try one set for fit? provided you buy something 
eventually! I ended up with 'mid-seventies on' exchangers from Denmark on my 
'78, they didn't have fittings for the booster fan hoses however.  tim s.