Well it has been a long time since I posted any progress. I finally got around to modifying the shortbed to fit my highboy frame. I created a how-to thread in the body forum. Here is the link:
See that thread for more detail, but for the sake of continuity I wanted to add some of the build progress to this thread.
Here is my makeshift gantry for testing the bed fitment.
I moved the middle crossmember to match the highboy location.
I made a new crossmember because it felt like too big of an unsupported section and don't want the floor to bend.
I made new holes in the bed for the narrow frame. I used sleeves and filed the holes square for carriage bolts to work. I welded the old holes closed.
I made several standoffs to adjust the bed to the different frame contours
I fixed a few rust holes on the floor
Then finally I sprayed the underside with Zero Rust and the inside with EZ Liner.