[Tech II]

Well, you obviously have a leak somewhere, which is creating an airpocket....fact that it overheated, is not good.....

Tell your "real" mechanic, it is impossible to install a waterpump, or thermostat backwards in this engine.....

Have you put a coolant pressure tester on this?
for your sake, I hope it's external and not internal(like lower intake or head gaskets).....