[Sirrotica]

If it has a serial number it has to be legal to drive on the road. Mule cars normally didn't have serial numbers, or titled, and were destroyed by the manufacturers upon the end of their usefulness, and test use. They were never meant to be sold to the public, used for engineering exercises.

Between 1976 and 1977 the PMD engines were saddled with much more smog equipment because of the change in the clean air act. You can't just stick smog equipment on any engine, they all have to be certified requiring dyno time, and road/track testing to be certified by the EPA for on road use. Certification is expensive too.

In 1973 Pontiac tried to slip a ringer camshaft in the super duty 455, they got caught by the EPA and were required to switch to a milder, more efficient cam midway through the 73 model run. Two ratings for the SD 455 in 1973, 310 HP with the illegal cam, and 290 HP with the EPA cam. I believe that was rather costly for Pontiac when they got caught. The SD cars were a very limited production car in 73, only 252 cars were made, and they still got caught.

When there were no/little smog rules (60s) it would be much easier to slip a different drive train departing from factory OEM than after the EPA came into the mix. If Pontiac got busted for deviating from EPA regulations, it would be both expensive, and damaging to the division. Some heads would roll if the EPA got wind of a car like described. The juice wouldn't be worth the squeeze.

If you think the EPA doesn't have teeth, and influence, just reflect upon what happened to VW diesels when they cheated on their software on their engine management system, it was very damaging, and costly for VW.