If you put a wiring harness for the truck brand new into it, I am not sure how its wired as far as what is connected to the ammeter. If I wired it from scratch, NOTHING would be connected to it. Alternator-Ammeter-Battery. In your case, you want to remove the ammeter completely, so Alternator-Battery. But, later on, a wiring harness may tell you to connect something to the ammeter post! I believe if that ever happens, they are asking for unswitched power.
In any case, a Voltmeter reads from Ground to Battery positive and you can wire that up anywhere. If I were doing it, I would find Ignition ON positive (same place you wired your HEI), wire the positive end there and the negative to the case of the meter itself. That is with the understanding that the meter housing is grounded to the dash. In that case your meter will not read anything until you turn on the ignition.
