Just a couple of thoughts on your problem, you may have one or many problems, So I will name a few to help the best I can, If you have changed the Radiator, water pump, and the plates and seals inside with upgraded ones, you may want to start next by Jetting up your Carburetor, todays fuel is not made for Carbureted Cars, everything is Fuel Injected now, a Lean condition will cause your car to run hot, if that's not the case, then you may want to switch out your Distributor to a Mechanical Advance MSD drop in unit, set your total timing to 36deg at about 2500 RPM, again timing can cause your engine to run hot, something else to look at, you may have a crack someplace in the heads or block, maybe even a blown head gasket causing a problem, allowing compression gas to enter into the water jackets causing it to heat up quickly, that one I had first hand on a 350, would heat up in 15min, coolant smelled bad and had some black stuff floating in it and with the engine running, cap off the radiator bubbles were seen coming out easy way to tell, was not letting water into the oil, or using water, but just allowing compression gas into the water jackets, if that is the problem, make sure to flush the block out along with the radiator, sediment in your water jackets could be a problem also not allowing the coolant to flow around the cylinders enough to remove the heat, check your Fan make sure it is on right, and if you have a clutch fan make sure that it is working right, make sure you have your fan shroud on, a must for sure, Pontiacs tend to run warm as it is, a shroud makes all the difference. If I can think of anything else I will post more.