Kevin,

Is this the rig that sat under the tree for many moons? I looked at a wiring diagram of a '61 GMC. I don't know if the Chevrolet has all the same details, but there is a 6 way wire connector located on the drivers side firewall. Could this have corrosion or mouse crap in/on it...

If your rig has a ballast or some type of ignition resistor, I don't see how that would involve the instruments in the dash. It looks like that connector powers not only the gauges, but also supplies power to your ignition toggle switch.

If your rig has these connectors, you might pull them, clean them and shoot some type of WD-40 or something on them. It's worth a try.

Stuart