hi, according to NHRA, tech, the 315,675, 302 are the legal heads allowed.
the newer 302 head, has larger port volumes with 1.78 &1,50 valves.
this is for 1965 only. the 1966 273 is allowed same heads plus,920,585,973 castings. I would call NHRA tech, get a written ruling on the allowed heads,
If you want to be competive in stock, have the heads done by a shop, that knows and does stock eliminator machine work. also, have them do block machining. in the real world of stock racing, you need someone that speaks stock eliminator. most important areas are, pistons, bore finish, valve job, ring combo, balance,cam grind/lobe shape. most average shops do not know or understand how to make a stocker run fast. you have to think outside the box, so to speak. welcome to NHRA stock elimator. a friend , just built a 235/273 for I/SA, so far has run, 12.20's on a 12.65 index first time out.