voltage across battery when charging ?

[82 GT]

Now that you have a ground to the frame, is there one more alternator you can try?
That will tell for sure if the PA alternator is bad since you are now getting 13.3v with a different alt.
Sometimes an alternator with a bad diode will not always show up on a test bench.
I would try one more alternator, if you can, and if you get a good reading I would send the PA alternator back to them and let them test their own unit.
The guilty finfer seems to be pointing at the PA alternator. It sounds like you have it pretty much narrowed down to the problem!