See if you can hit up a junk yard first. IIRC there's a section of frame just spliced in to lengthen it for the long beds. There's been at least one thread on here where someone's made a long bed into a short... Maybe they still have the pieces? Good luck!
I forgot. In addition to the 14" mid-section you will also need to add just over 4" of channel to the rear. It's a PITA to get everything line up for the receiver hitch/bumper.
I'm wanting a long bed so bad I can taste it, can't sell mine, dealers only want to give 26 for it, I'm thinking really hard about making this truck a long bed, just hate to change a clean truck.