New motor.....

I would double check your timing, retarded timing will cause overheating. also new motors do tend to run hotter, it is also possible that you have an intake gasket leaking and pulling oil into the intake ports, a vaccumn check will show this. if the vaccumn gauge reads steady then most likly its not leaking, if the vaccumn guage is not steady chances are you have a gasket leaking, you didnt say at what speed the car was being driven, but open road is always best for breakin.....50 -60 mph.. .. at lower speeds a fan shroud will help if you dont have one...