[Suomien]

Aren't there all kinds of "rules" regarding what accessories are present during testing to determine HP? Whether there's a belt-driven alternator, water pump, nowadays even the A/C compressor might be included.

Back in the mid-60's, it was all engine, no accessories, and "special" carbs even. Back then, they probably tested with race headers even (remember the picture of the Cammer with the glowing headers?). Now, measurement is more standardized, so that the numbers actually mean something. Theoretically.