The water pump fittings will be reading the temperature of coolant returning from the radiator, which will be a lot cooler than the stuff leaving the engine. Get a thermostat housing with a 1/2" pipe fitting thread in it, and install the idiot light sending unit there. Then install the regular temp sensor where it belongs, at the back of the head.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!