Bulkhead Wire Question

First, I'd use larger wire than 12, more like 10.

IF you mean remove the original battery feed terminals (inadequate) and just "sticking" the wire through the bulkhead connector, you can remove the connectors and ream the holes slightly, and just feed it through

IF you are bypassing the ammeter, going to a voltmeter, this means you now have TWO free connections, the old battery infeed and the old charging line from the alternator. NOW what you could do (IF you want to retain the connector) is to find replacement connector pins, replace them, and use TWO with no12 on each, IN PARALLEL to use as your main battery feed.

You "could" retain your stock ammeter, but the way Painless and other aftermarket systems are set up, you need to do some custom wiring, and unless you DO feed wire through the bulkhead, OR build a "remote shunt" ammeter with the shunt in the engine bay, YOU WILL STILL be faced with the same original problems with the bulkhead connector, as here:

http://www.madelectrical.com/electricaltech/amp-gauges.shtml