aftermarket volt/alt gauge wiring help

You are right (one wire is ground and the other 12v.) and you could connect it at the old amp meter terminals behind the cluster. (best place in my opinion)
Do NOT connect it like the amp meter was though, as you want the positive wire for the gauge to connect at that spot and the ground wire for the gauge to be directly to ground.

Or right off the main large power supply wire to the column (ignition) would also be fine and to ground with the ground wire.

The positive wire needs to be on a switched power connection in any case.