51 GMC still overheating

I had a 51 GMC (with transplanted 235) that was giving me fits with the thermostat not opening up till the engine was at 215 degrees, then it would open and everything was fine. This only happened after the engine cooled down below the thermostat temperature.

Turns out that the heater is used as a bypass when engine is cool. The return heater hose fitting at the thermostat housing was completely clogged with rust. Cleaned it and never had the problem again. Go figure.