Here it is mounted in the dash frame.

Rear view.

After I cut the hole in the frame, I used electronics breadboard to fill the gap and mount the voltmeter to. It's strong, a good insulator, and seemed like just the ticket.

I drilled and tapped the potmetal(?) cluster frame for 6-32 and held the breadboard/voltmeter assembly in place with some socket head screws. I ran out of washers, hence the missing one.

The voltmeter tilted upward when it was first mounted, so I slipped two washers behind the breadboard to get the angle right.

It looks a little sleazy, but it works. Long term, I need to do something better than that, I just haven't figured it out yet.

Looks good so far. Thanks for sharing!

Someone PM'ed me and reminded me that I never posted any completed photos of the ammeter in the car.

If I had it to do over, I would at least investigate if there is some way to print clear overlay decals at home, and I'd copy the face of the voltmeter onto a decal and apply that to the old ammeter face. Or, it is possible that a company that does vehicle wraps could make such a decal, using the original voltmeter face as a template. It would look better that way, but I'm probably not going to tear it apart at this point and re-do it.

Thanks for the update, Jim. It doesn't look bad. Most would probably never notice sitting in the car. I'd call it a good upgrade.
I moved my battery to the trunk with a cutoff switch a few years ago, and thus had to cut the alt. wire feeding the ammeter, and route the alternator power back through the cutoff switch so it actually kills the car. Thus, my ammeter now only shows draw. I'm sure there is a way to get power from the switched side back to the ammeter to make it function properly again, but I feel like if I'm going to get under the dash and do that, I might as well upgrade to a volt meter. (that's what brought me to your thread.)

Thanks again-