[James]

Did you used Teflon tape? If you did it could interfere with the grounding of the sending unit. You should not use Teflon tape on the sending unit, use a very light coat of gasket sealer on the thread. If you have an ohm meter you could measure the resistance from the brass base of the sending unit to the battery negative terminal, with the ignition key off. It should not be more than a couple of ohms. If that check good I would suspect a bad sending unit. If more than a couple of ohms then the ground connection between the water manifold and the head could be at fault, I'm assuming you had the head off. If you do not have a meter I would put the old sending unit in and drive it to see what is happening. You should be able to switch them with minimum lose of coolant.