From what I have seen the water pump can make a difference but not so much the housing. Good idea to try the pump first.
Also, what is the condition of the original 3 core radiator? If not recently rodded out or re-cored that may well be your problem. It sounds like the rest of the system is in good shape.
After I replace the pump and that doesn't make a differance the next step is the radiator. When I got the car the seller said he had the radiator rodded out. Of course that was back in the late 90s. The car didn't run until I got it. It may have gotten something in there while sitting around.
