Craig, those kinds of senders should read Full: 29-33 Ohms, Empty: 244-250 Ohms. You can use an Ohmeter at the sender to check if this is the case. GM, Ford, and Chrysler senders are opposite Stewart-Warner and are different Ohm values. If the sender shows 240 Ohms or so when full then perhaps the sender is upside down on its mount in the tank. I wouldn't think reversing the wires at the guage would make any difference as you found out. Good puzzle!