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I sort of did what you are talking about but I made a long bed shorter than a short bed. Took a 20 something inch section out between the front of the bed and the wheel well. It went back together pretty well. It's also a good time to put in a new front bed panel if you need one. Don't know where you want to shorten it at but in front of the wheel well is pretty strait, behind the wheel well the bottom of the bed angles up and makes it harder to work with.
Go on Spike TV and watch the "Trucks" episode when they cut up a Chevy. I think it was their Class Six truck(or which ever one was the 63ish one). I have seen others do it and their way looked way more professional and safe.
Ya need to Z-cut the frame btwn the rear cab mounts and where the frame starts to slope upwards and then fishplate the frame. It'll require removal of 16 inches (regular cab).
The bed will also need to be sectioned so the well is centered over the wheel along with moving the frame crossmembers.
Bob the bed? Doable... but ya don't gain a whole lot cuz of the location of the rear spring mounts. Again, you''ll need to map the cut to preserve the dent contour and the rear side marker light.