Is your truck running and driveable? The cheapest (just to get it rolling) is probably a used set of wheels and tires off Craigslist. Then once you get the truck running and stopping and driveable, you can figure out your exact plans for it and get the right tire/wheel combination to fit your end goal. You would hate to by new tires for a project and if like me, it takes a fwe years to finish and now your tires are a number of years old, maybe have some overspray on them, and are starting to deteriorate!
