I fabricated some height-adjustable "legs" which I welded to the frame, so I could set the frame absolutely level. Or, at least as level as a 1961 stick-welded truck chassis is going to let me.
I removed the four forward bed mounts because the 12" chop doesn't coincide with any of the bed frame placements (I'll be drilling a couple new holes), and I cut 12" out of the frame sides (vertical cut because that's all the room I have), measured from 7" behind the cab mount.
The trailing arm crossmember pieces were cut free, as were the forward legs of the X (I mis-measured though, and had to add some metal back. By my new measurements it looks like you'd need to cut out 11.25" here, but I might be wrong); Pythagorus failed me.
I tried to heat the X to bend it to where the stubs needed to line up, but it was taking forever and in the end I just sliced them with a cutting disc and welded them back up as needed.
8" was cut off the end of the frame.
Clamped it securely, checked for square, and tacked it thoroughly. I'll weld it up when my arms and shoulders are cooperating with me again.