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Even though its great your stopping by for future refrence you'll probablly get better answers in a forum devoted to 1978 truck.
I don't think it is your water pump, my first step would be to flush your heater core. I'm assuming if you replaced a junkie radiator, then some of the grunge from the old radiator made a home in your heater core.
This is the forum you need, the 1973 - 1979 F-100 And Larger F-Series Trucks: https://www.ford-trucks.com/forums/forum38/
One more though, double check all your heater controls.
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.