Where is my coolant going???

[muttruck]

This truck is my son's. It was given to him so there are many unknowns. I sent him to school with my car. It runs reasonably well, but the engine is tired. I have discovered the engine is about a 1977 305. He replaced the intake manifold when he first got the truck. I have discovered I am loosing coolant. I have checked the water pump for leaks tightened all of the hose clamps and never see coolant on the ground or frame, the oil is not foamed or discolored. The radiator was fairly new when we got the truck. I am pretty sure he sealed around all of the bolts into the heads. Any ideas?