Wiring a voltmeter is easy. Since you are negative ground, connect the negative lead to ground and the positive lead to a green or white wire. A standard 2" (or some call them 2-1/8") gauge will fit the original ammeter hole. You'll probably want to wire it's light into the dash lights (also easy).
But don't be surprised if it doesn't quite indicate actual battery voltage. The original ignition switch has a bit of a voltage drop.
You can keep the ammeter if you want. If you remove it, you'll need to join the two or three wires from the back of it together securely.
It's actually not difficult to make the original ammeter work correctly with an alternator, but you'll have to wire it differently than Triumph rescue said to do. I did my first conversion over 30 years ago and it worked just fine for me. Search the archives for "shunt".