[Captainfab]

Doing an axle flip on a dually will likely lower the rear a bit more than 4". I would thing going with air ride would get you where you want to be easier than messing around with leaf springs. You would likely have to build the majority of the mounts and arms. You could use the front spring hangers as an attachment point for the lower arms. I have seen similar kits for the squarebodies.