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I'm hoping for some electrical advice. I have a '54 3/4 ton pickup that has negative ground and is 12 volts. According to what I've read, both of these must have been converted from original. I just found a fuel gauge at a wrecking yard and I don't want to fry the thing. I have a resistor, so my question is: do I put the resistor in the red line or do I put it in the black to compensate for the polarity change in the system. Any help would be greatly appreciated.

I don't think the polarity will make much difference in the operation of the fuel gauge, but excess voltage supplied to it just might cause problems. Most gauges are "balancing coil" type, with one winding receiving full battery voltage and going to ground, and the other coil getting battery voltage and completing the circuit through the tank unit with a variable resistance. You really need a variable resistor in the input side, that can be dialed in to supply 6 volts to the fuel gauge while it's in operation. A 1,000-ohm potentiometer works well, with one end of the resistor connected to 12 volts, the other end grounded, and the variable tap connected to the gauge input. Start with the variable resistance maxed out, with almost no voltage supplied to the gauge, and dial it up until you get somewhere between 6 and 7 1/2 volts to the gauge. Lock the potentiometer shaft in that position with a little Loctite or even Elmer's glue, and forget it! (This needs to be done with the gauge hooked up to the tank unit, and the gauge grounded to the dash. It won't work without a good ground to the gauge housing!
Jerry

The 12v system is a conversion, but the negative ground is only different if the truck is a GMC. AD trucks: Chebby = neg. ground, GMC = pos. ground.

Thanks gents for the advice. I've got the gauge working, now I just need to deal with the tank sender unit. Its either not grounding or broken--1 step at a time. By the way the truck is a GMC.