The 51-6 gauges are thermal or bimetal type and are not grounded except thru the sender. There is a voltage regulator on the back of the cluster which does need a ground and gets it through it's mounting screws. It is a rectangular can with 2 terminals centered roughly under the speedo. That supplies power to the temp and gas gauges in your case, and gets its power from the fuse labeled instrument cluster. Check for 12 volts there. The output is a nominal 5v but is a rapid pulsing voltage of 12v, 0v, 12v, 0v in an on/off cycle, the speed depending on load which gives an average of 5v to the gauges. Your meter may not be able to read that very well. If the regulator has failed and supplied a permanent 12v to gauges, they may be damaged. If 0v out and fuse is good, then regulator needs replacing. If you have a voltage on the regulator or "in" terminal of the gauges (voltage in side of gauges are tied together with a short jumper) coming from regulator, then by grounding the other gauge terminal, the gauge should move. DO NOT leave the ground attached--just keep it on long enough to see if gauge moves. If they do, then do the same thing at the sender end. If no movement, check wiring, if gauge moves you need sender. If no movement with the voltage present from the regulator, then gauges.
There is a service manual and most technical bulletins on site you can download or a wiring diagram only is here
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