Ammeter to Voltmeter...who does it?

Sorry, Logan you are simply incorrect as regards the Mopar ammeter. Take a good look at the picture I posted. The entire "meter" consists of nothing more than a brass shunt across the terminals, with a moving vane attached to the needle. There is no coil.

In order to get it to measure "volts" would require it to draw it's full scale amperage.

In other words, if you WERE to convert it to read voltage, the end result would draw what the ammeter drew at full scale, say, 60 or 70 AMPS

I don't know about you, but that's not a very good way to build a voltmeter.