Alan, I'd move the engine temp sensor to the manifold and the fan sensor to the bung on the lower radiator tank. Your Walker should have a bung in lower tank on the engine side. Also change your thermostat for a 180* unit.
I do not agree with running a carbed engine hotter than 180 if at all possible. There are too many variables involved in proper combustion and timing for an electronic or points ignition system to deal with. In a modern engine the computer has the ability to vary these very important settings (fuel/air mixture, timing and others) based on the engine's requirements so it can run more efficiently at a higher temperature. Most non computer engines can't. I believe your mechanic is correct, you shouldn't run your engine any hotter especially with added cooling requirements such as air conditioning.
Just think of how your engine's cooling system works. How hot or cool your engine runs is based on the capability and efficiency of your cooling system but the thermostat regulates all this. If its running too cool then your thermostat isn't regulating the temperature and has failed in the open position. If it runs too hot then either the thermostat has failed closed (or nearly closed), the cooling system has a problem, or there is a mechanical reason inside the engine that is responsible (blown head gasket etc). I think if you change your temp sending unit (buy a new one they aren't expensive) and relocate it to the manifold (should be a bung up by the thermostat housing somewhere) you'll get a more accurate reading.