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

[mclairmo]

First, you really need a shroud to ensure all the air being pulled by the fan goes through the radiator core. Not only does the water pump need to circulate the water through the engine, you also need the radiator to efficiently transfer the heat out of that hot water. Also, use a hi-flow T-Stat and make sure it is opening at the correct temp by putting it in a pot of water on the stove. Use a meat thermometer to take the temp of the water. Also, make sure that your water pump pulley is noticeably smaller than your crank pulley. Otherwise, you will be spinning the water pump too slow and that could also cause a low speed/idle cooling issue. Make sure radiator is not clogged.

[dougminton]

You said it was a "new" 350 could it have a reverse rotation "new style" water pump that's being turned the wrong direction? Just something else to check.

[El Jay]

Are you getting an air lock in your water jacket when the thermostat's installed?

I always drill a couple holes in the T-stat flange to guarantee a water flow. (That's an old T-stat I had lying around)