Blowing head gaskets is pretty rare on these rigs (as far as I understand), was there any weird temperature stuff going on before it happened? I would expect severe overheating before a head gasket going, particularly on a 2003.
You mentioned having the water pump and timing belt done early in your build, if you had a shop do it I would investigate a bit more because they may have buggered something up that caused the overheating and eventual head gasket. This might result in someone else paying for the new engine.
Alternatively it could have been bad from the PO, but I would imagine you would have found it by now....