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I am not real sure of a cheap way to be honest. Just in replacing parts that need to be on mine, is pricier then that of a SWB. You have the font hubs to deal with. I know that many places sell spacers, which you could pick up and install. It wont be quite as dually looking as the real thing.
The biggest expense will be the rear dually fenders. I found them new for about $280 a fender, used are a super big challenge finding. Most salvage yards dont have them, and the ones that do, they are broken. Horrible fiberglass product!
A flatbed would be cheaper in many aspects. I have heard many here talk about that kind of conversion, and that the SWB box and frame is more narrow then the DRW box and frame.
I don't think the pickup frames are different between SWB and DRW. The cab and chassis trucks though are different. The axles are narrower and the frames are narrower also. If you can just get an axle from a DRW pickup truck you will be fine it just bolts in.
Find somebody that want to go from duals to singles and swap. That's be the easiest and cheapest. Other than that, Dually got it right about the dually flares. If you're going with a deck, then it's easier. The frames are the same between the single and dual and as Neal said, the Chassis cab is narrower at the mounts. As an interesting side point, the chassis cab rear axle I've got will fit pefectly on my '52 F3!!!