The reason the horsepower numbers drop off after 1972 is because the auto manufacturers went to a different rating method mostly to satisfy insurance companies who were well on their way to finacially killing the muscle cars with high horsepower ratings. The old method rated gross horsepower at the flywheel and after 1972 they used a net rating that was calculated differently.
Since most of us will use at least a mild performance cam and other upgrades the numbers don't mean much if you want to use a later model block. The cyl head chamber volumes came in a few sizes so head selection is more important than the year of the lower end.
