Ammeter to Voltmeter...who does it?

The actual needle movements are the same for an ammeter and a voltmeter. Each responds to a tiny amount of current applied to its coil. Too much current and the movement is toast.

If it is applied as a voltmeter, there is a relatively large (value- not size)resistor added inside the case- in series- that resists current flow to the movement thereby allowing a tiny current to flow through the coil. Size the resistor right so that the guage reads "12v" when connected to a fully charged 12v battery and your volt meter is good to go.

If you want an ammeter, you will connect a very low resistance in parallel with the coil movement so most of the current will go through the resistor and only a tiny amount through the meter movement. Again, it is calibrated by using the right size resistor to read amps.

If I were to convert to a voltmeter, I'd grab a few voltmeters from fords or chevies next time I was at the junk yard, break them open, isolate the resistors and measure their values with a digital ohmeter. Then, break into the Mopar ammeter, remove the parallel shunt resistor and add the junk yard resistor- or radio shack equivalent- in series and maybe even outside the meter case if it was easier. Change the 'A' to a 'V' and go cruzin.