Cut the frame behind the cab, you can tell were you need to cut, bolt your short bed to were it needs to be on the back half of frame then fix it to were the bed meets the cab good, cut frame to length and brace it and weld the hell out of it
Not sure is this will help because its an older truck but on youtube go to thedieselstop.Wayne has done a few long bed to short bed conversions.Hope this helps.