JFYI: Pontiac lowered their compression in 71 like everybody else did but they did it differently than Mopars. They used the same pistons, deck height, gaskets, etc... but they opened up the head chambers to much higher cc's. I used to know the numbers but it's been so long It's hard to remember but I believe the older heads were around 70 cc's and the newer ones were as high as 100 cc's. Even though you read the compression was rated at 10.75 to 1 it's quite possible their ratings were flawed like Mopars were due to machining tollerances. According to my compression calculator using a stock bore 400 with the piston at zero deck, a .040 thick gasket, 10 cc valve reliefs, and a 70 cc head that only figures up to 10.21 to 1.
Your correct about the heads,the problem is with 400s in paticular the pistons are down in the bore factory (.045-.060),they also have something like 12-14cc valve reliefs,the good h.p. small heads are 72cc,the later heads are all over from 84 to 114,and there are a few early in between heads but they are usually low pefromance press in studs.