Ammeter to Voltmeter...who does it?

Since you have the cluster out, there's a few things you should consider doing, no particular order
1...Did you do some sort of ammeter bypass, other than just bolting the wires together?
2....Cluster is "not very well grounded" OEM, by the mounting screws. Add a grounding pigtail from a common point on the PC board and bolt to the column support or dash frame
3....If you have not replaced the gauge regulator/ IVR, this would be the time
4....Check the harness to board connector pins and repair / solder as necessary
5....The spring fingers that form the socket for the gauge IVR should have jumpers soldered across to the board traces
6....Gauge stud nuts can get loose or corroded. Loosen/ tighten them several times
7....Obtain a rheostat or test resistances and check gauges accurasy
8...Certainly the time to replace and do maintenance on lamps. Clean the board where the sockets contact, eraser, etc, and clean or replace sockets, consider replacing bulbs. Some guys like LEDs but there can be problems with the dimmer on them.