Thanks, Hacke !!Found an interesting page:
Recharged: Rebuilding a 1963-’68 Chevy Alternator and Adding a Solid State RegulatorWhen Chevrolet introduced the redesigned Corvette in 1963 one of the new features was the Delcotron alternator that replaced the venerable generator. Dubbed thewww.motortrend.com
Great information, and it looks like the same alternator I have!
I've got some reading to do now, then when I get back out to the farm I'll do some testing on what I've got to see if I can figure out what the issue is: wiring / voltage regulator / alternator.