Alan, I would move your temp gauge sensor to the location where you're fan sensor is now. This should give you a more accurate reading.
If your radiator doesn't have a bung for a fan thermostat, I would spend the $30 it will take (check here
) for an adjustable fan thermostat that will replace your radiator petc0ck. This adjustable switch will allow your fan to come on and go off at whatever coolant temperatures you desire and it's the way most street rods run electric fans.