89 b220 engine help needed ASAp

b2200 engine help needed... ok i start my truck and it does the usual tap.. ill change the head and fix that.... the problem is this... I drive my truck like two blocks and the radiator cap blows clean off? the water is boiling hot... the fan works... radiator isnt clogged.... what could it be? i took thermo out and still its hotter then the devils fire...

blown head gasket, cracked head, or cracked block. That much pressure means it is a pretty major leak.