Can't get more than 12.5 volts thru charging system

That might be but it' d be helpful if we actually fixed the problem before going into the theory of a B-29 electrical system LOL
We are helping people understand the gages in a '67 Dart.
Chrysler figured drivers were smart enough to have instruments instead of idiot lights. They are more useful once you know what they show.
A lot of people don't know what the ammeter shows. The fact Chrysler often labeled it an alternator gage only adds to the confusion now.