Yes, I have it setup as 6 volt negative ground at the moment. It was 12 volt negative ground when I bought it but all the wiring was shot. I rewired using the factory harness and went back to 6 volt but stayed negative ground. Everything works fine except the fuel, temp and oil pressure gauges. The temp gauge only goes to the middle when first starting and the oil pressure will work for a few seconds. Fuel barely moves. No voltage reducers installed. If I put negative to the sending units, the gauge needles move all the way across but are very jumpy doing it.

 

So, if you have crud in the tank, you'll never solve the reliability problem until that is fixed. You can't filter away the accumulation at the bottom of the tank, in the pick up tube, and now in the line in front of the filter.

As for the gauges, the easiest way to test them is with a 9 volt battery - the kind often used in smoke alarms. Should give you a quick read on condition.

 

I've never heard of recommending a 9V battery to test a 6V gauge. The Ford Shop manual (pg 275) states to test the gauge, supply it with 1.5 Volts DC and it should read about 1/2 way.

The stock sending unit uses the The King-Seeley (Ford Stock) principle. These sending units supply a full 6V through the heater wire in the gauge in the form of a square wave with the duration of the "on" being the variable determining the gauge reading. The longer the duration, the more fuel in the tank. It is this full 6v of current which heats the bi-metallic strip in the stock gauge that causes the gauge to read via a small gear train (that may adjustable?). Most after market sending units are simply a variable resistor which sends a constant voltage to the gauge, lessening that voltage as the fuel level goes down. Therefore, the sending units signal doesn't heat the bimetallic strip in the gauge as much. Unfortunately, the calibration of the after-market sending units and the stock gauges don't match up very well due to the differences in the principle of the signal.

 

Nor had I until I asked a fellow who refurbishes vintage gauges. He knew about the Shop Manual but said the 9 volt battery was his go to method. It worked for me after getting nothing from the Manual process. Go figure.