Ammeter to voltmeter conversion

I'm sure this has been discussed here before, but nothing shows up in a search. I would like to convert (or should I say replace) my ammeter in my rallye dash with a voltmeter. I have seen it done on other Mopars, but none of those were A bodies. I can remember an article somewhere that explains it in detail, including the make and model number of the voltmeter needed. I can't locate that article, either. Can anyone here help?

Thanks!