Somebody correct me if I'm wrong but although a set of 6x, 16, 13, or anyother head made for 67 and up 400, 428, etc...., they will bolt onto any 65 or 66 389, 421. The problem with doing this is if you run a high lift camshaft you will have problems with valve to piston clearence. If I were to rebuild a 389 I think I would use my stock heads have them ported and polished and haveby Scott Sheeder - Pontiac GTO Forum