@bucket my K30 SRW has 3.5" wide brake shoes and so do DRW axles. He hasn't stated which drums he has. I know the 3/4 ton SRW only have 2.5" brake shoes and I would assume narrower drums. If he installs the hubs from the DRW axle to his SRW axle (assuming it has the 2.5" wide shoes), will he need to change the backing plates too?
The brakes aren't the issue, but the flange on the axle housing is the same place no matter the width of the drums, it's the backing plate that takes up the difference.
A DRW 14ff has hubs with the wms inset, and the housing flange is inset as well to accommodate the DRW style hubs.
A SRE 14ff has hubs with the wms more outboard, and the housing flange is more outboard to accommodate the SRW style hubs.
Both DRW and SRW 14ff's share the same overall width from AXLE flange to AXLE flange. The DRW 14ff ONLY has a narrower track width due to the HUBS.
Putting 2" spacers behind the inner dual is basically the same thing as just running the SRW axle that was already in the truck to begin with, as far as track width goes.