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There were several changes that affected the HP.  Of course the change in rating makes the comparison look wonky but the actual HP did change a lot as emissions controls took over.  Using the L48 engine that was available in the Chevy Nova as an example.  In 1970 the HP was 300 and compression ration was 10.25:1.  In 1971 the compression ratio was dropped to 8.5 and HP to 270.  In 1972 they changed to SAE rating which was 200 hp without much change in the engine itself.  In 1973 the HP dropped to 175 mostly due to the retarded timing, AIR pump, and EGR used to drop the combustion temperatures.  The last year for the L48 was 1974 when it was rated at 185 and future cars had Cat converters and were unleaded gas only.
The decrease in octane and lead content in gasoline was the major factor in dropping compression ratios.  In 1970 Sunoco 120 octane was $0.25 per gallon in eastern Oklahoma.

1973 was the first year for the AIR pump, and compression ratios were being lowered because of UNL gasoline.  It still was better than the latter 70s and early 80s.  Back then you had a fast car if it could break 10 seconds getting 0-60.  I recall my Nova did it in 8.5 seconds, my little GTI today is around 6 seconds.

That would make a neat crate engine.  I'd put it in a Miata.

That would make a neat crate engine.  I'd put it in a Miata.

Even if you can fit it, that would be terrible. It would be undrivable, not fun at all. 

1958 Pontiac Bonneville

Lawson's Motor Wheel, 1902