They're cast, and 12 PSI is a lot of boost for anything this old of a stock bottom end. Be sure to give yourself lots of extra ring gap or else the rings will come too close together and break the pistons when the ends make contact while still expanding. A rev limiter wouldn't go amiss here.. are you computer controlling the air/fuel/timing situation?

1965 C60 school bus | 1967 GMC 6500 school bus