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. The radiator and fan combo along with the engine tune-up and condition will dictate the operating temp. The thermostat is nothing more that a temp controlled valve that aids in warm up time and how soon the engine gets to operating temp. I would run a 180* T-stat all year round.