Modern electronics = Full rewire

There's an old thread on here that has lots of good ideas for ammeter/ voltmenter. Also read the MAD ammeter article, and don't necessarily "do it their way"

https://www.forabodiesonly.com/mopar/threads/ammeter-to-voltmeter-who-does-it.119480/
http://www.madelectrical.com/electricaltech/amp-gauges.shtml