You can cut loost the brackets from teh axle tube that hold the backing plates on, and move them in towards teh diff by about 2". Then you can run dually brakes on a SRW axle.
A SRW axle is narrower than a PICKUP TRUCK version of a DRW, and a CAB AND CHASSIS dually rearend is NARROWER than a SRW pickup version.
So from sidest to narrowest....
DRW pickup
SRW pickup
DRW cab and chassis.
The only way to make it work is to cut the mounting brackets for the backing plates loose and move them in....or make new ones as a two piece design and weld them on 2" closer to the diff.
Why do you want to do all that extra work for an extra 1/2" of brake shoe width?
I THINK, you can machine out the 3" rear drums to acept the dually 3 1/2" shoes....do't quote me but I remember a thread about this.