Wiring in a volts gauge.

Yep. you want switched igntion, which is a little difficult to access, as it does not go through the fuse panel, OR you can just use switched accessory, which means the meter will be on in "accessory" not a big deal. Just get into the back of your box, find a spare connector, and run an inline fuse with about a 1A fuse off to the meter.

Here's a thread about mounting them right in place of the ammeter. In that case, you can solder to the IGN trace on the board --the one supplying power to the instrument VR

It's a long thread, and covers at least three types of clusters

http://www.forabodiesonly.com/mopar/showthread.php?t=119480&highlight=ammeter+conversion