It would be much simpler to find a short bed truck, than to try to shorten both the frame and the bed. I've owned and managed body shops, and build race car frames from scratch, but I'd be reluctant to begin a project like that.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!