So it has no 3rd and 4th gear, or it's skipping from 2nd to OD (4th) gear?
If it's the no 3rd and 4th gear, that typical when they go, as you put it, tits up
My first 700R did the same thing in my 67. Cruising down the highway, all of a sudden the rpms shoot up RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR, when I slowed down, it would shift to 2nd, then when it was time for 3rd, RRRRRRRRRRRRRRRRRRRRRRRRR.
Sorry man, that blows, but with 225K, assuming it's the original trans, that's way longer than most 700R (or did they have the early 4L60's?) last. They're a good trans when taken care of and not abused, but hell, they're trucks, all they do is get abused!
Truck is looking great though!!!!