It's commonly stated that pre-net engine ratings (1971), that manufacturer's often overstated HP ratings.
It's important to know that OEMs also UNDERstated many engines, notably their top-tier engines, since they were regularly competing against each other in sanctioned events.
The NHRA invoked 'factoring' that re-calculated HP ratings after verified stock cars performed 'too well' for their ratings. The Chrysler 426 Hemi, rated at 425 HP, was immediately refactored to 480, and the FoMoCo 427 'Hi-Riser' was refactored from 425 to 465.
Noted racer & car tester, one Roger Huntington, did his own calculations on the Pontiac 421 SD. Rated at 405 HP, he deduced via it's performance times it was putting out at least 450 HP.
Something to keep in mind before 'poo-pooing' vintage ratings.