72 Slant Six running too cold

180° is the correct choice of thermostat rating in a car like yours. Don't know exactly what you mean by "pointy side out", but the thermostat gets installed so most of it, including the visible coil spring, is inside the head -- not protruding outward into the water neck. The heater valve has nothing to do with engine temperature regulation. Sounds to me as if the resistance of your temperature sender has probably drifted off spec, or the temp gauge itself has a problem. Does your fuel gauge read accurately? If so, then I would replace the temp sender and see what happens to the reading on the gauge. Also unplug the bulkhead disconnects and check for corrosion and loose sockets; clean and correct as necessary.