If the 72 heads have been rebuit and the 76 haven't, i'd use the 72s. the 72 is probably a 346. I believe some of those had hardened seats. The 76s(452 )will for sure.Heads from 72 to 78 are all the same performance wise. I believe the need for hardened seats has been exaggerated. Seems like GM had a much bigger problem with valve recession than Chrysler did.