if you pull the dash out you might as well make some jumper wires and hook it up to a 12v battery. use the terminals on the dash that the wiring harness plugs into.
12v to the gauge feed, ground the dash through one of the bolts or something i think...then see if you get 12v to the voltage regulator and 5v out of the reg. you will see the circuit trace where it runs to you yr fuel gauge...that should carry yr 5v.

i hooked up some resistors and grounded the other side of the gauge with ~80ohms, 25ohms and 12ohms. i think that should be empty, half tank and full.

also you can use yr ohm meter to check the resistance on the wire that signals your fuel gauge, see if that reading correlates to what your gauge was reading...