I don't believe this will work, you don't have any way to retain the axles. The 8.75" uses retainers at the housing ends, and by going to 8.25" Jeep housing ends you don't have that anymore. And the Jeep axles are c-clips axles, which the 8.75" third member doesn't have provisions for. While you might get everything to fit together, you wont have anything to keep the axles in the housing.
Keep at it though, ideas like that might lead to other ways to get it done.
The real issue with running the Jeep rear brakes is the thickness of the bracket not letting the axles touch the thrust button. If you took the thickness difference between the disk brake bracket and the drum brake backing plate, you could figure that into the axle length, if you were to have a set replined anyway, or make the housing a touch shorter if you were going to narrow a housing.
Hmmmm...except that doesn't fix things if the Jeep disk brake bracket doesn't locate the caliper correctly because the offset from the end of the axle to the housing is different.
Personally, I'm not a big fan of the swap anyway as the rear disks are solid. I want vented disks.
I've been planning to make a set of rear disks off a late '80s Supra work, myself. They had an internal drum brake emergency brake so I can go to a fixed caliper later, they are a vented rotor and the bolt pattern is a match already (holes are too small though, and the center register is way small). Just need to figure how to modify the e-brake backing plate to get it back far enough to fit the rotor, and then build a weld on bracket to locate the caliper. Been to busy with the 13" cobra rotor/Corvette caliper front brake kit to work on it though.
