What head should I get for a 318

Your thermostat has noting to do with operating temp. The thermostat just regulated how warm the water is in the block and heads before it goes to the radiator.

Thats is the operating temp.
The thermostat has everything to do with how warm your engine gets. In the summer, running without on can overheat the engine. The thermostat does indeed regulte the coolent in the engine. But thats the operating temp your looking at.
Other parts of the engine maybe hotter, but if the coolent gets to hot, it can cool the rest of the engine well.

The radiator and fan combo along with the engine tune-up and condition will dictate the operating temp.

Well if there in poor shape, so is the runnig condition. Seperate the first 2, rad and fan from engine tune and engine tune-up and condition.
A bad flowing rad and/or fan will overheat the engine quickly. The coolent gets cooled in the rad. How cooled it gets should be cool enuff for it to enter the engine and cool it down and keep it cool long enuff for the replacement fluid that came out of the engine to be suffeciently cooled to cool the engine down and continue the cycle.

Engine tune-up and condition will add to the problem unless servere enuff to be it's own menace.