It the broader picture over years of results that you need to look at. That is the beauty of looking at the mountain of track data.

Certainly you can find a PSMC 426 car that ran 126 mph and a 440-6 car that ran a 106 mph, or a 440-6 car that ran a 118, and a 426 that ran a 111.

But when you toss out the outliers, and look at 20 years of data, the PSMC hemi cars ran 4 to 6 mph faster than the 440-6 cars. That is probably indicictive of a 40 to 70 hp difference. In the early years, they were all slower. In the later years, they were all faster, but stayed about the same relative to each other. In a more true to factory stock, the relative difference was probably 3-5 mph and 30 to 50 hp.

I personally trust this high level observation as a reflection of what is generally true when comparing relative stockish 426 to 440-6 motors even though none of these motors racing in PSMC are stock. It is certainly better than a singlar dyno test compatison using motor parts laying around the shop.