As the ZR2 uses the same size 265/65/17 tire as the other Twins, there should be space back there to do what you want. I'm kind of curious now if the ZR2 has different part number fender liners, as the ~2" width on either side should fit differently.
Problem is you either have an ExCab long bed or a Crew Cab shortbed, rather than a longbed Crew, which would be ideal. Might be easier to get a regular Twin and a ZR2 rear. You'd end up with a selectable rear locker in that scenario, something not available on regular Twins.
Tire size is actually 255/65/17 per the Build & Price. (This is the same size tire as other models of the truck)
Specs appear to show the body width is 76.7" instead of the 74.3" - I suspect that could simple be wheel flares.
Tread width is 65.9" as opposed to 62.4" - I assume that tread width is the centerline of the tires. That is 3.5" of difference. The dimension between the wheel wells is 44.4" - to get to 48" of width, you need 3.6" of additional width, plus if you add my spary in bedliner idea, you probably need closer to the full 4" to make room for 4x8 sheets. In my eyes, you have dug into the safety factor the designers built into the original design, assuming that the tire and not part of the suspension was the driving force behind the design.
May be premature on my part, but I think this idea would be pushing the envelope a little too far.
I think you need good comparison pictures of the regular and the ZR2 wheel wells: If the ZR2 design was not only pushed out the extra width of the tread, plus the geometry of the suspension was slightly altered, then maybe it would work. You need those pictures loaded and unloaded to really understand the interactions.
Can anyone state how far the bed drops when you load 1,000 pounds in it? Or 1,200 pounds in the bed? Is it 2" or is it 6" of drop? (At 2" of drop with 1,000 pounds, I would argue all of our trucks could have been designed with a pretty squared off bed with no wheel wells intrusion into the bed. )
I don't load heavy loads often, but next time I do, I want to look at how much the rear of the truck drops.