1976 Chevy truck not blowing hot air it has new thermostat and water pump?

Do you have a temp guage? How is engine temp? The new thermostat is good but realize it could be stuck open. If you have a guage you'll be able to see if the engine is ever heating up. They can be tested by dropping them into boiling water and watching them. Radiator clogged? If so you could be running hot but no water flow, thus no heat. I would think it would have to be pretty clogged to get no heat at all, maybe enough to overheat, not really sure. But if you are truly running borderline hot due to no flow get the radiator taken care of before you lose your head gaskets on an overheat. A really weird one happened to me once - 1984 Renault Fuego Turbo - ran the turbo really hard on day that was almost -30F (below zero) - forced so much cold air into the engine under full turbo boost that we watched the temp guage drop to rock-cold - at 100 mph on Rt 80. Hour later - turbo was shot. Engine ran so cold the turbo lost lube and bearing overheated/warped. Hope something here helps.