Never messed with the "new" body style but 97 and downs used a 37" wide frame rails on the trucks that came with a bed as far as F250s and F350s. The F350 cab and chassis used a 33" wide frame rail. If the same holds true for the 99 bodies and up (I believe it does but don't quote me) then you can find a rear out of a F350 truck that came with a factory pickup bed on it. The c/c axles will have the spring perches welded on 4" narrower than you will need. The wheel spacer kits are available to fit the duals onto your axles but I think they look a bit weak. The front end will also need the dual wheel spindle spacers if you want to run budds on the front, or you can just leave it alone and have two spare tires.
Up to 97 factory c/c trucks used a 137 wb as opposed to 133, believe the 99s and up started using 141 for c/c and 137 for pickup beds but I'm not 100% sure.
Hope that helps.