Happy ending
Well it was a tiny bit embarassing, but I found out yesterday that the new Prestolite alternator is fine. The problem was apparently my eyesight and my troubleshooting technique.
I had installed the gray brush/slipring cover they gave me in place of the integral regulator. In the process of tightening the terminals on it, I changed the orientation of the connectors underneath so that they shorted. Even though I was aware of the possibility and checked visually several times and "seeing" they were not, they were indeed shorted. I didn't bother to remove it completely and use the ohmmeter on it.
I hope I didn't hurt the regulator. Again. Possible sad story there.
*** Update - Regulator is not damaged, even after shorting across the field wire twice.
No charge for that and a quick output check from the folks at Romaine Electric in Seattle (on Sixth Ave S near the ballparks). They have a big starter/alternator rebuild operation. I recommend them as capable and kind to DIY folks like us. When I brought my ancient 55 amp to them for preventative maintenance, they recommended a new one since it would be cheaper. It was a really good deal because they credited me for the core exchange (~75 bucks) and then handed me back the old alternator. It needs some cleaning up - and I bet I could get new brushes and bearings from them and rebuild it myself if I wanted to. If the bearings are press-in I bet they'd do that for me. Now I have a spare alternator to carry around.
Years ago, when I had more time than money, I did that with a starter motor that had worn-out bearings. I did the disassembly/reassembly and a rebuild shop kindly sold me bearings and put them in the frame for me for nothing.
I'll remember the wiring checks recommended by Ryan, if I need them.