Jeep rear discs on an 8 3/4 axle solution

JR.,
Heres what I love about this site, People actually ask
intelligent questions, at least some do:-D
Your question was good enough to make me stop what I was doing, clear the bench and re-assemble the brake assembly.
Because this brake set up was intended to be bolted to the outside of the tube flange under the bearing retainer, it does not have the clearance to be moved out towards the axle flange the required 3/8".
Boy that would have been easy!!!!

You were not thinking wrong.
The drum portion of the disc would hit the bracket and the parts for the emergency brake would not have had room.
Great thinking though.
Andrew