Hey there, new to this site and boats, picked up a 72 Sanger with a built up 455, not sure what's all done to it but dude claimed 500hp and it was built by Rex Hutchinson out of Sacramento.. Took the boat out once and she blew a head gasket, started leaking water at the mating surface.. Tore it down over the weekend, had a bad valve looked like it got hit by something, no piston damage.. But between the valves there were cracks in the heads. They are "c" heads, found some "g" heads that I'm gonna get checked and cleaned up, anyway what do I need to do to the new heads to get them ready for a boat? I've been reading about enlarging the oil drain back holes? Is there anything else I should do? Thanks