As was said volts is volts, don't mater if 6, 12. As long as the meter is made for more then the highest output from the source.
google search;
https://www.google.com/search?client...volt+meter+car
The meters shown are going to be bigger then stock A meter. The depth should be ok if mounted on the face of the instrument panel like the A, but the diameter of the face is larger and flange even more so. I don't have one in front of me and it's too late to go out to the shop a check.