The easiest way to replace the ammeter with a voltmeter is to disconnect the two ammeter leads, bolt them together VERY tightly, using a Nylox nut or Loktite, then cover the whole thing with double-wall heat shrink tubing. You could also cut the lugs off of the ammeter leads and solder them together. How ever you decide to do it, just make sure the connection is clean, tight, permanent and well insulated.
The voltmeter can then be connected to any convenient switched +12V wire.
If you don't want the voltmeter to look totally different than your OEM Veglia gauges, Don Byars sells a voltmeter that looks very similar. If you want it to look like it's OEM, North Hollywood Speedo & Clock can convert your OEM ammeter to a voltmeter. They can either leave the OEM ammeter face on it or replace it with a very OEM looking voltmeter face.