Severe voltmeter needle oscillations

Also you replaced the engine side spade connector in the bulkhead but did you tighten up the female side of the dash harness in the bulkhead?

Honestly no. The female side of the bulkhead connector, on the dash harness, was (and still is) in a very good shape.

If tightening the I/S connections doesn't resolve the problem, I will have a look there.

It would be ideal to find an OEM ignition switch connector like this one :
IS order.jpg
The best way to improve my I/S connection

Maybe I'm going to restore a good connection with an aftermarket male/female connector.
This implies installing a connector onto the I/S switch, but also onto the main wiring harness.
Connectors.jpg

or this one (maybe easier as the wires are already plugged in) :
Connectors 2.jpg


François