Amp gauge vs volts gauge?

If you are going to run a very big alternator, say 50A or larger, you need to experiment with a shunt across that one. I'd start with a length of about 10ft of no14 wire, with eyes crimped on, and just wire it across the ammeter. You can trim it if need be. Shorter makes the ammeter read more amps full scale.

If the 6V system was positive ground you'll have to revers all wires on the back