I know that the change from net to SAE rating systems knocked the numbers way down.

The compression was dropped significantly but I don't know how much of an impact that had on the engine output.

Did the cams change?

I've got an old Motor repair manual that shows a slight change in timing.

Anything else change?

Looking at the old road tests it seems the 1971 and 1972 Imperials ran about the same times (slow as molasses) with the 90 hp rating change and a swap from a 2.94 gear to the 3.23s.