gauges gone wonky

If this happened immediately after changing the switch, I'd suggest it is probably something loose in the wiring, bad connection, etc.

But I'm not familiar with the wiring in those early cars.

Check the IGN feed out of the switch, should be one wire break off to feed the cluster. IF it's "y'd" off right at the switch, it might be a poor connection there

You might have bumped it loose at the cluster. Do those use a connector on the PC board? Typical failure, as you might have pulled on the harness enough to disturb it.

It MIGHT JUST be time to pull the cluster and "go through" it

In my case, there were several problems which I had to fix

Several of the PC board connections were broken, and there were ground issues. I ended up eliminating the original connectors, and went to Radio Shack and bought some "Molex" style connectors

The nuts on the gauges may not be making connection, due to corrosion, even though they seem tight