Update:

I had vacuum. It was easily detected by putting my finger over the hose at the modulator while the car was running.

In talking this AM to the shop where I bought the modulator, the stuck pin is normal. Locktited in place in this vintage.

Peter's comment about hard shifting being a consequence of a failng modulator struck a chord. For a day or two before the tranny started slipping, and for a cause at the time I attributed to a poorly set kick down linkage, the car did exhibit much harder shifts, one more noticable was a decided thunk when dropping back into second at a stop. And I was actually "getting rubber" on upshifts when accelerating agressively.

Cranking the new modulator's key in a few turns and then driving, cranking some more, driving again etc. caused VERY minimal improvement.

So...what I now think happened is the modulator finally failed causing hard shifts, and lockup was so dramatic it fairly quickly (while I was focused on the overheating/radiator problem) caused something to "give" inside the tranny.