Im a little confused. You have a 2wd? And you dont want the dual wheel front? You can simply replace the DRW front rotors with SRW rotors. The caliper stays in OE location, with OE pads.
The 2WD DRW rotor is basically an extension anyway, just get a set of SRW rims.
It's called a phantom dually.
If you went with a 4x4 knuckles, you'll need new calipers, bearing assy's, brake hoses, and somehow a pair of grade 8 bolts to torque the bearing together.
Why would you downgrade the rear axle?
If you're putting SRW wheels on the rear, what're you gonna do with the rear dually fenders? If you take off the DRW extensions, you have huge, ugly, open fenders. They're cut out larger than the 2wd flairs.
Or you're buying a new/used pickup bed...
Honesty, it'd be cheaper or easier to sell the dually and buy an actual SRW truck.
As for the rear disc conversion... why? Theres nothing wrong with rear drums and, when adjusted properly, the brake pedal feel is better than 4 wheel disc. They last longer and the parking brake is MUCH more effective.
In order to swap in 4 wheel disc, you'll need a new master cylinder, install a proportioning valve, and new rear steel and rubber hoses, brackets. Then there's the swap kit, rear axle seals, gear oil...