The early 70's "915" heads are slightly better than any of the open chambered iron heads but would not be worth seeking out only that if they were on hand and in good shape you would want to use them. There is a closed chambered iron head but I cant think of the number right offhand. If you can port I would find Steve Dulcich's excellent porting articles for stock iron open chambered heads & he got killer flow. You mentioned stock rebuild and what is your CR (OE actual measured SCR might be in the 7's). How far down are the pistons? That is what is going to hinder you the most. I would port the hell out of your current heads and call Dave or Kevin at Hughes for a cam to build some compression