So here's the follow-up:
I took the truck on the trip, about 21,000 pounds total between the truck and the trailer.
By the time I was 300 - 400 miles from home the torque converter would barely hold lockup, it was very difficult to drive it without frying the trans.
I hated the transmission, and still hate it even just driving to work and back.
I'm gonna yank it out and buy a Firepunk Street & Track trans. By the time I buy a good converter, a valve body, bands, apply levers, flex plate, etc. I'm more than halfway there anyway.

 

Further follow-up: the Firepunk trans is going to happen but it got put on hold because my engine has a 53 block... sure enough, it cracked. I managed to find a nice 1999 Mexican block to rebuild it on, it's almost done (I have a friend who owns a diesel truck shop), then as soon as I can scrape up a dollar or two it's getting a Firepunk.

 

Follow up to the follow up!
The Mexican block motor is in and rather than order a Firepunk I had Ed's in Marysville, WA do a full-tilt, billet-everything, triple-disc transmission for it.

Now it works.

 

Good to hear. Probably got just as good a transmission for a lesser cost.