where did the 70s take the horses

I'm sure someone can post the equation for HP gain (or loss) per point of compression, and that will also confirm that it's about a 5-10 HP loss if both engines are measured on the net scale and maybe 20 if measured on gross.

We've already established that there was no radical cam change, so the only other major areas are heads and compression.

I'm not aware of any huge head mods from 68 to the early 80's except for hardened seats.
I think we are also all aware of the mid 80's 302 heads.

That leaves the compression and that's simple math.

OK, maybe also exhaust. HP vs std manifolds have been discussed ad nauseum and the consensus is also 5-10 HP. Not sure of the flow differences in the castings in the specified years.

I will call attention to the factory 73 318 with dual exhaust (Roadrunner) which the factory thought was good for 20 HP (170 net vs 150).