Why do I have coolat shooting out of the radiator?

Installed a used engine into my father in laws truck (I'm already very skeptical). While the engine is running, the coolant is sucked way in, so I top it off. Put the cap on. When I shut the truck off, coolant shut out the secondary overflow like nobodys business. Also boils into the overflow tank. Also note: made the mistake of leaving the cap off and about a gallon and a half of water shot out when I shut off the truck.

My first guess is: blown head gasket. I understand air bubbles in the coolant while running, and I understand maybe some coolant pushing out when I shut it off, but not like this. But sucked in while running, then blown out like crazy when shut off (i.e. Cap off whole time)?

Note, the heater inlet/outlet is higher than the rest of the system. I have tried to fill it, and turn on the heat, but every time I open it's filler port, it's near empty. Is it possible the heater is still one big air pocket pushing it out?

1997 Ford F250 HD w/7.5L.

Added (1). Ok found the problem… And yes.Monster air bubble. The thermostat wasn't opening, and didn't have a bleed hole like most do. So when running, the water pump was pushing water against the air bubble, and when shut off, the air would push water back out the radiator.

MONSTER air biscuit! Turn heater on HIGH, if necessary park truck at up angle to assist burping cooling system. Also, make sure a thermostat is installed. If that doesn't work, pressure test cooling system and test for combustion gases in coolant, that would SUCK to have installed an engine w/a blown head gasket.