3A heads suck. You want #4's, #5's, #6, or #7. There is a set of #4 on ROP right now. If I were you I would buy those. Very hard to find heads. Drill them to 17/32 and stick your 403 valves in them. Get springs to match your cam. Get an adjustable valvetrain. The guys on ROP
www.realoldspower.com can get you set up. I am olds403 on ROP....Olds