Gt maybe I'm wrong the little box be hide the Alternator has a wire going to the Amp gauge now he's running volts no need for the wire, run the Alternator wire to the battery.
Most of the time the amp guage just has the charge wire going "through it", there is a shunt on the gauge back that actually has all of the charging current going thru it and to eliminate an ammeter, technically you would hook the two wires (supposed to be orange and red) together. good to go...The ignition wire then goes to (+) on the new voltmeter,
I dont think anything needs to change at the alternator level.
A single wire hookup alternator is fine, but normally not the best way to go IMHO, the best thing about them is the simplicity, the negative is the fact that it charges to "battery voltage and not systen voltage, bnot a big deal, but if you get into stereo's and high draw ignitions and in our case a lot of bilge pump draw, you could undercharge the battery and run more amps than neccissary (as the volts drop the amps go up)
Just a matter of opinion, I have a single wire on my jet, but a three wire on the motorhome and the I/O...
GT