Back when I was in college with the '66 Newport, when it was about 7 years old, one day I got in it to go somewhere off-campus. At a red light, I smelled smoke and noticed the Alternator gauge was in discharge and would not charge as it normally did. Took it to the dealer and they replaced the wire going to the gauge, down at the bulkhead connector. Poor connection, they said. No more problems.
Unusual for the gauge to spike, though, as if the regulator might have stuck of something.
Download a FSM from
www.mymopar.com, then look at the wiring schematics to see what wires go to the gauge itself, inside the passenger compartment and also under the hood. Then look for heat damage to the wiring. Hopefully, that should lead to the problem.
Keep us posted,
CBODY67