Excellent question, Jim
12v negative ground.
You can run a big "+ power lead" cable to the starter post, and the heavy wire from starter post to the ammeter.
Some people have taken the guts out of the voltage regulator, and then run the heavy-wire from the starter to a post on the voltage regulator box, and the heavy-wire from the voltage regulator box to the ammeter. That is the "neat-looking solution", using original wires..
Before I read that idea, I use a bolt/nut connection near the old voltage regulator and carefully/thoroughly wrapped it with electrical tape.