If the motor was replaced recently and you are finding "no" coolant leaking or blowing out the tail pipe, it might be possible you have an air pocket in the coolant system including the heater core. Before pulling the head, maybe try to bypass the heater core, check the core for blockage, verify no heater hoses are vise versa etc. Also, not knowing the H2O pumps condition you may want to toss one in to rule that out. Who knows, if the pump was on the engine and it sat for a period of time, the fins may be deteriated and can't blow the system to specs.
Good luck !!