where did the 70s take the horses

Right but pure stock there is a big difference. I also bet the cam is much less aggressive come mid 70s but that also is often changed at the same time as intake and headers are added.
The gross to net changed because the federal government required it, but it became a convenient excuse when selling cars when HP numbers did drop because of other mandates in emissions. The air pump was another big power robber those years that just pulling the belt off fixed.