Update: Problem solved. The alternator, VR and Voltmeter are fine. When checking grounds, etc earlier I simply glanced at the connections on the starter. This morning, I looked at it again, the connections appeared clean, the nut felt tight to the touch but looking closer, the split lock washer was not fully flattened, the nut could be tightened further still. Mystery solved.

I had the car load tested at Checker and it now shows a healthy charging system. When I start the car, the voltmeter in the Tach goes to just above 14 and stays there the whole time (drove it about 40 minutes).

This problem didn't surface with the old alternator. I can only guess that time and vibration took their toll. I've only owned the car about a year and never laid hands on the starter until now.

Thank you to everyone who posted suggestions and ideas.