The manifold isn't "White hot" Objects take on a dull red glow around 1100F but cast iron melts at 2200. You need to do a cooling system pressure test and check the cap for starters. As mentioned, make sure the heat riser is open when the engine is warm. Like Rusty mentioned, a non contact thermometer can be an effective tool to check block and hose temps. Say that the block near the thermostat is 230 F and the hose going to the t stat is 100F and the engine has warmed up, you probably have a stuck thermostat.
