The gauges run on a nominal 5v so you either need the stock instrument voltage regulator or some have managed to replace the regulator with an inexpensive
12 volt input to 5v out power supply available from Amazon or ebay. I don't know personally how well that works but worth a shot if you don't have a regulator. The one I linked to should work 1 gauge satisfactorily for testing. If you use the stock voltage regulator, in additon to the power in terminal and the output terminal, be sure to ground the case or it will not work properly. Depending on the supply voltage, a non working regulator could damage the gauges.
There is an article on testing the gauges in the Dec 1950 issue of the Service Counselor
https://packardinfo.com/xoops/html/downloads/SC/SC-VOL24NO12.pdf but the two sender resistance values you need for 51-56 gauges are 70-75 ohms at empty and around 10 at full.
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