There are also issues surrounding how much exhaust backpressure is set on the dyno when doing an "advertised power" run. Now, it has to match the backpressure in the vehicle. Back Then I think they just ran with straight pipes essentially like a marine thru-hull exhaust. Also the "SAE Gross" horsepower involved running without the accessory drive, whereas today's power run must have the entire accessory drive operating. We must consider the effect of tetraethyl-lead from days of yore, which is a free octane gainer 3-5 points IIRC with nasty environmental cumulative damage. Easy HP to be made on a pushrod engine by cranking up the compression ratio and adding spark advance. Lots more tricks today with direct-inject, coil-on-plug ignition (to eliminate the distributor drive), oil pumps that have variable flows, piston cooling oil jets, aluminum heads to take away the knock limit with knock sensors to avoid destroying pistons.
Advertising a high horsepower today is mostly about technology with the valvetrain to allow a higher rev limit. Just produce an average amount of torque higher up the RPM scale and there is horsepower waiting for you. The classic equation is HP = TQ * RPM / 5252, with torque in foot-lbs, so you can see mathematically is a higher RPM is a multiplier.
I was involved in pushrod engine development about 20 years ago, many things are the same but obviously with new technology. The race/push for fuel economy has really opened up a lot of new additions to the classic internal combustion setup.
