I did it, don't remember exactly what I used. It's been a really long time. It was likely a cheap Sunpro like that one, the face certainly looks similar. Here are pics of mine:
I just shucked the gauge and swapped it in place of the ammeter in the gauge cluster. Just make sure the studs from the new voltmeter do not make contact with the traces on the original board.
I actually did the oil pressure gauge at the same time, replacing it with a mechanical gauge.
Your '85 cluster is going to be a little different from my '83, but it should still work, I would think.