I'm not aware of any DC conversion kit to do what you're talking about, and although there are aftermarket kits available, they still really do not cut it. Most of the aftermarket kits are nothing more than spacers to adapt dual wheels and bolt on fiberglass fenders. They do nothing to upgrade larger rear ends, axels and brakes.
There have been others who have found junked 3500's and or found donors willing to swap. I think you'd be much better off going that route than messing with a kit. I considered this for a while before realizing it just wasn't worth it and bought a 3500.
Kev