changing the gear in an 8 3/4" is as easy as pulling the axles out a little and removing that entire center section you have pictured and replacing it with a complete center with the desired gear already in it! Then push the axles back in and adjust them and you're back on the road! A reasonabley saavy mopar guy could do that procedure in a couple hours!
If you just want to change the gear in your center section, that involves disassembling the entire center, removing the pinion gear, ring gear, crush sleeve, and putting it all back together with the correct lash on the gears on the crush sleeve preload, not an easy rehab!! I guess you could get the new hard spacer and shim it, but you still need some pretty extensive knowledge about rear assembly!