Hi all - getting the car fixed and getting home was my priority so I didn't follow up here. But here's the whole story :-)
The shop I found was Eurotech; Rick and Janis and Cliff there were SUPER helpful and went way above and beyond to help me out. I also got to meet a really nice group of Smoky Mountain chapter PCA folks, but that's another story.
Anyway, as I said the alternator in the car was relatively new, so I suspected that less, imagining some other 28 year old part would be more likely to be the issue. Still, no other simple problems like connections or ground surfaced. However, once the required shop opened on Monday morning, Eurotech sent the alternator to be bench tested, and sure enough it was bad. The problem seemed to be one of the white leads - the middle one - not soldered correctly to the capacitor at the end. Simple, but a needle in a haystack. It
looked fine.
Rick helped me be sure to get a new alternator under warranty, same day, and got me on my way by 3:30/4:00, and I immediately drove to past Indianapolis, slept, and then to my home in MN, 1001.6 miles, without issue. Rolled the trip odo right past 000.0.
If you are in TN show these good people some love
Eurotech Knoxville
Rick is very capable and knows these cars. I was really lucky.