Rebuilding my 390 in a 67 Mustang after 20 years. My question for you is it worth upgrading to stainless, undercut valves? If it is, then I would need to go to larger valves as I can't find s/s intake and exhaust in stock sizes. I ported and polished way back when and don't want to switch to way to expensive heads if I'm not going to gain anything. Your input would realy help!