Are we talking the SAE GROSS era 60's stuff here? Because yes, there was some serious neutering that happened in the 70's, but there was also the transition from HP being measured in SAE GROSS to SAE NET, which changed the figures considerably. There was also some crossover there where we saw the same engine measured in NET instead of GROSS before all the smog stuff happened.
LS6 Chevelle
Motortrend, with some prep, managed to get 13.44 @ 108.17mph, curb weight is 3,836lbs
https://www.motortrend.com/features/1601-flashback-road-test-of-a-1970-chevrolet-chevelle-ls6/
Indicated power: 460HP, 500 lb-ft
E39 M5
Car and Driver got 13.3 @ 108mph, curb weight is 3,976lbs
https://www.caranddriver.com/review...benz-e55-amg-2000-jaguar-xjr-comparison-test/
Indicated power: 394HP, 368 lb-ft
Same trap speed, M5 is 113lbs heavier, so that old 454 was probably around 390HP-400HP SAE NET, when adjusted from GROSS. Still fantastic numbers for the time, but down 60-70HP when corrected.