Let me guess, the first time it overheated and then blew a head gasket.
Bad move.
Common problem, The cylinder head is made of heat treated aluminum (after it's cast it's heat treated). Once you get one REALLY hot you anneal the metal and it becomes MUCH softer than it was originally and it therefore no longer stable. Also more prone to threads pulling out etc. Anyway most machine shops never test the rockwell hardness of heads before they work on them, nor do they reheat treat them. So even though the head is flat, it's no longer dimensionally stable and it will repeatedly blow head gaskets. I've learned most of the time if an engine overheats to the point of blowing a head gasket with an aluminum head, it's best to go find another head than spend any money in vain attempts at repairing it. I'd suggest you do the same unless you enjoy replacing head gaskets! :-)