1972 chevy 1/2ton pickup overheating, not sure why?

[chevman51]

I just purchased my brothers 1972 chevy pickup that he was restoring. He put a brand new chevy 350 in it that's built pretty good with cam and so on. The pickup will overheat just sitting there idling in the driveway. I pulled the thermostat out and the gauge will stay down. I put a new thermostat in and the gauge just creeps back to the hot again. I looked at the water in the radiator with the truck running and it looks like its circulating so the water pump should be working, right? Where else do I go from here?

Thanks Tony