Pontiac are infamous for heat problems well here is the why it happens. The engineers at Pontiac designed the cooling system that works well as a system, however if something is changed then it doesn’t function correctly anymore. Like, changing the Cam or heads which are changed on my car. What they did is an old hotrodders trick they use an underdrive pulley on the crank. Meaning the crank pulley is equal to or smaller the water pump pulley.
The fix, I fixed this on my car by looking up the part numbers for the pulley of an AC car which are 9790842 and 9790843 for the crank and 9788886 for the water pump. I could not find 9788886 so I used 9796061 with a ½ inch spacer. 9796061 is for a 4.5-inch water pump and 9788886 is for a 4 inch. The original had an 8” water pump pulley and a 7.25” crank pulley. So, let’s do the math. 1/(8/7.25) x 850 rpm = 770 rpm @ the water pump -VS.- The new pulleys are 6.5” water pump and 8” at the crank. 1/(6.5/8)x 850 rpm = 1046 rpm @ the water pump, a difference of 276 rpm. It no longer overheats at stoplights.

Sounds good. The trade off is more drag on the top end