If you can source a SWB truck, as CrazzyWolfie suggested, you're way ahead if the game, as you get other things such as E-brake cables and drive drivelines that are already set up for the shortened wheelbase. Then, cab swap your frame and bed back to the other truck, and with some sweat equity, you might get away with even breaking even, after selling the truck with your old LWB frame and bed.
OR, you CAN shorten factory stuff. Truck would be down longer, but its 100% do-able. If you are a competent (truly, honestly competent) welder, you're ahead if the game. If you had to pay for all of the welding, it's a money losing proposition! I've done three trucks. A '59 Chevy, a '66 Chevy and a '68 GMC. it's not horrible. Body work is the majority of the actual work. Heres my '68 GMC (wrong brand, I know) part way through the process. This truck got shortened on both ends of the bed, as well as tubbed 4" per side, to make room for drag slicks for. You can see the "scars" where the tbed was shortened. I can show how I shortened the frame, but will have to take some photos. This type if work is fairly universal across brands. Step sides are easier than fleet's! Fleetsides get a lot more labor intensive!