If you have any salvage yards/ auto wreckers in your area, go there and look for an 86-97 F350 dually truck that left the factory with a pickup bed installed. The rear axle will be a direct bolt in on your truck. Those axles are 4 inches wider from brake backing plate to backing plate, so the tires will clear the springs. Make sure the gear ratio is the same as your front. The same yard may have the front dually hubs for the Dana 60 so you can run the same dually rims on the front. This is the way I'd convert a single rear wheel truck to duals. Arrowcraft makes a conversion kit, but they use spacers, not ideal in my opinion. They also sell the rear dually fenders for your box.
1985 F250 Reg. cab 6.9,94 factory turbo, zf5 speed, 3.55 4X4, flatbed, dual 3.5in. stacks
1990 F350 SRW Crewcab 7.3 turbo 4.10 zf 5spd 4X4
1997 F350 SRW Reg cab powerstroke 4.10, zf5spd 4X4, 3inch downpipe, 6637 mod, intercooler
1996 F350 Crewcab dually 4.10, E4OD, 3" downpipe, 6637 filter
1996 F350 C&C Dually reg cab 4X4 flatbed Zf5 4.10, 3" downpipe, 6637 filter