Racing junk is you best bet, buy a truck someone has invested 3-4 times what he is asking for it, make a road trip out the inspect your purchase, (could be a good time in itself) then drive the truck home.
When you start adding things up like an angle iron here, tires there, in-dash stereo, some chrome wheel simulators perhaps, then toss in all the time you could/should be working on your race car.
Right now with the used market so flush with end of season rigs, and the economy so depressed there is pile and piles of trucks in the under 20 grand range.
If you don't have 20 grand to spend on even a schoolie conversion don't start! The bus will cost you 5-10K then a few hundred here, a thousand there and you'll be at 15 17K really fast. You'll then have a schoolie that you could probably sell if your luck, 8-12K when your done.
Sorry to throw this at ya, there is a reason there are not a dozen online forums for truck conversions, other than the pros, its just not worth doing yourself right now.
2001 GMC 6500 Topkick, 22' box, dropped frame, designed to fit into a 9' garage door. 3126 CAT 6spd Man Lo-Pro 19.5's w/ 3.07 rear axle ratio