I have been tinkering on GMCs for over 50 years. They are famous for having "glass heads" because they crack so easy. I have learned over the years that if a head shows any sign of a crack during magnaflux, get rid of it. Cracks do open up wider when they are hot. I have tried everything from KW block weld to water glass to having them welded and nothing worked. And having coolant in the oil is a quick way to ruin your bearings and cam.