How do you fix a cracked head in the engine?

Depends where it's cracked. This was a common problem with the 6.2 Detroit Diesels that GM used from 1984 - 1993. You'd have to magnaflux the head, find the cracks, drill a hole and tap it, fill it with a bolt, cut off the remainder of the bolt, then smooth it all down. Not someone you really want to "learn as you go" on your own vehicle.