Re: changing amp reading to volts?
The ammeter is a zero center meter, so it requires some manipulation to position the needle to the left zero position. The wiring is pretty easy, just ground the negative side, install a 18K resistor in series with the positive lead, and the meter should read about 18v full scale. If you are really concerned with accuracy you could put a 10K series resistor with a 15K trim pot to dial it in with. You could leave the amps scale and make 12v the midpoint so that it looks like an ammeter.
"For those that will fight for it...FREEDOM ...has a flavor the protected shall never know."
Semper Fi! L/Cpl Edwin L. "Tim" Craft, B Co 3rd AT's, Khe Sanh Combat Base, February, 1968