Fuel/Temp Gauge Accurate w/Lights On...??

Your cluster likely has grounding issues. There were several problems with my 67......

1......Several of the connector pins were loose, or even broke off. I elected to solder pigtail wires to the board traces, and connected them with a pair of Molex connectors instead of the originals

2......My IVR was bad, but even the SOCKET connections were bad. Solder across from the PC board traces to the contact fingers

3.....Two or more of the gauge studs/ nuts were not making contact with the PC board. Loosen/ tighten the nuts a few times to 'scrub' the connecton. Better yet replace the sheet metal nuts with "real" ones

4....GROUND.......the gauge cluster LISTEN UP!! This is likely your problem..........only grounds through the mounting screws. Solder/ bolt a pigtail wire to a common ground point, and screw that to the dash frame or column support

There's some tutorials around here on sprucing up the cluster board............Here's one now

Printed circuit pins repair