For what it's worth, several years ago had a load shift in my truck when I had to do a hard-stop. Pushed the front of the box tight to the cab in the center of the box.
Didn't break anything, but the bend was something I couldn't let go. Concerned ice would build up and break rear window. Truck was only 1 or 2 years old.
I essentially did same with two ratchet straps; backed up to a tree and started on outside, took progressive bites toward the center until it was pretty good. Used a short bracket I made of angle to spread the tension, even out the pull. I just used the ratchet, didn't try move the truck.
There was still a "dimple" in the center of the top rail.
I traded that truck years later, and still see it occasionally around town....I can spot it by that dimple...