I had the same issue, my battery was good, but when running I would only get a reading of around 11V across the battery terminals. In my case it turned out to be both the brushes in the voltage regulator and the slip rings in the alternator. Here is a link to my earlier post with some photos and how I "fixed" it. Not sure if an 83 has the same alternator as my 86 but it is probably similar.
temporary alternator fix - cheating