I don't know all the specifics here but some of the changes (all done for emisions) were switching to unleaded fuel (hardened valve seats) lower compression by using dished pistons. Other changes were advancing the cam two degrees, great for top end power but lowers torque. I wouldn't do it in a street engine, but as I said, emisions. The boss 351 motor lost its solid lifters, got a milder cam(I think so on the cam, not sure) and became the 351 HO. That was about it for internal changes between 71 and 72 that I can remeber. Someone else probably remembers a few more.
I don't know if they might have messed with the ignition timing.
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