I'm a '40s and '50s guy, so take this advice with a grain of salt.
You say "wires" running to the unit? Plural? Usually, there's only one wire going to an insulated terminal on the sender, which is controlling the ground resistance for the gauge. A second wire could be just a separate ground - my '50 AD has two wires, the second just grounding out on one of the mounting screws. A second wire could also be power for a fuel pump IN the tank . . . you'd probably be able to hear that pumping when you turn the ignition on.
Mismatched senders and gauges will usually read like offset scales: It will show FULL-FULL-FULL-3/4THS when the tank is full-3/4ths-half-1/4th, OR, mismatched the other way, It will read EMPTY-EMPTY-EMPTY-1/4TH when the tank is empty-1/4th-half-3/4ths.
But the only way I know of to find out the sender resistance is to have the unit in-hand and test it with a multi-meter. Then you can meter it and read resistance while you move the arm from empty to full and back.
You could: buy a universal sender unit at the parts store (you'll then know the resistance), brake the wire(s) going to your sender, and hook them up to the store sender to see how the gauge reads then. (don't forget to ground the sender body to the frame with a jumper wire.) That will tell you whether THAT particular resistance range is correct for your guage. Most common is, I think, 0-90 Ohms; other popular one is like 30-240 Ohms or something like it.
Then, I guess, you could put in a gauge of the other resistance range and see if it behaves with your tank sender.(and return the sender you bought).
OK . . .this seems complicated. How bad is it really to unbolt the bed of the truck?
-Michael
