From what I've read on the net of dozens people who've done this, you'll be time-, money- and headache-ahead if you just sell your current longbed and buy the body/frame style that you like.
The jobs are often over-budget and the results sub par.
Just get your current vehicle in sell-ready condition as cheaply as possible, be very patient, save up some money, and buy the best shortbed step/fleet side pickup you can afford. You won't be sorry.
-ftyler