The 64's no. The only "problem" is the 87 cc stock chambers. I owned 3 sets and they all cam in at 84-85 likely due to milling the decks and time. The smaller the chambers, the higher the compression. The heads you referred to (6X/5C/4X) are typically 110-114 cc with some coming in at 94 cc. Stock 67-69 Firebird heads were 72 cc with variances up to 75-77 cc for the 62's. The heads you are looking at would perform MISERABLY on a 400. Yes you can run 87 octane, but you would be at 180-200 HP. By getting the 400 to at least 9 to 1, you can run 89 octane,with 9.5 92 octane and 10 to 1 can be done with a well tuned carb and distributor.

You can put any 67-79 400 head on any 400 engine. Before you decide on a set of heads, just ask.. Been at a machine shop for years as a side gig and work on only Pontiacs. I do not know everything, but can certainly point you in the correct direction.