Sounds like coolant not circulating.
Sure the water pump is being driven?
Most are belt driven, either cam belt or alternator
belt.
Worst sceneario is that the head gasket has been
installed upside down, or back to front, you don't
say model/year. This could leave a water channel
blocked.
Just had, implies you didn't do the rebuild, go
back to whoever did it, surely they ran the engine
for a while after the rebuild?
Start be opening the interior heater valve, extend
the return hose to discharge into the radiator,
see if any coolant is flowing. If it is a simple
airlock this will do a lot of good.