Here is the definition of HP and when it changed in 71/72, I should have called it SAE gross HP (BHP) for 71 and older and SAE net HP for 72 and newer. During this time the insurance industry was demanding less horsepower from the big 3 automakers, some was reduced by lowering the CR in engines and some was reduced by reading HP with the engines accessories connected. I still believe your 1972 heads are the better set of heads than the 6X(8) heads, It would be a toss up if you had the 6X(4) set.
Maybe Mitch, Rukee, Eric, Jeff or some of the other guys will chime in,
SAE gross power
Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower (bhp), frequently referred to as SAE gross horsepower, because it was measured in accord with the protocols defined in SAE standards J245 and J1995. As with other brake horsepower test protocols, SAE gross hp was measured using a stock test engine, generally running with few belt-driven accessories and sometimes fitted with long tube (test headers) in lieu of the OEM exhaust manifolds. The atmospheric correction standards for barometric pressure, humidity and temperature for testing were relatively idealistic.
SAE net power
In the United States, the term bhp fell into disuse in 1971-72, as automakers began to quote power in terms of SAE net horsepower in accord with SAE standard J1349. Like SAE gross and other brake horsepower protocols, SAE Net hp is measured at the engine's crankshaft, and so does not account for transmission losses. However, the SAE net power testing protocol calls for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold.