Was the plate steel just welded to the angle strips? If so, it should be fairly easy to remove the angle strips. They were originally spot welded to the bed sides. On the outer side of the bed, run a sander along where the bed strips are (on the inside), then look for small dimples, which would be the spot weld locations. Then take a spot weld cutter Link
] and drill them out and the angle strips should come off (you might have to pry a bit if your cutter doesn't remove all of the weld.)
If the plate steel was welded to the bedsides as well, you'll have to grind the weld off.
Hopefully they didn't do that.
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