I'm thinking out loud here, but IIRC Magnum Beer Barrel manifolds have the sending units in different positions depending on year. Also, I think there is a coolant temperature sensor (black 2 two spade conn.)and a separate coolant sending unit (grey 1 spade conn.) Make sure the connectors aren't switched around and that either one is not broken. They do break easily and they are cheap to replace.

Search RockAuto for pictures.