I'd just leave the frame alone. Cutting it would be too much of a pain in the rearend. Just find you a good longbed and drop it on. Plus cutting and welding together a shorter frame weakens it and lowers the capabilities of the truck.
Can't argue the hassle of getting it right and proper reinforcement, but if again done correctly, the weight capability does not change. Semis and straight trucks get frame modifications done all the time, but the key is doing it right. Just cut and splice is not getting it right. You have to fishplate at a minimum. The biggest and most important factor is keeping it straight. If you don't get it back together right, you will make a permanent dogtracker.
Off Topic: This thread makes me want to build a Longbed flareside in this year model.
Would have to find a 8' flareside Bed from a 1973-1979, and put 1980-1987 tailgate, and 1980-1986 fenders on it. Or fabricate the bedsides to length. Would make people look twice at the vehicle, and have a cool factor of +10 to me.
If you wanted a Flareside and that's the reason why you want a short bed, AKA, the Red F-350 that the frame was shortened above, that would solve that problem without as much hassle.