The gears in a 8-3/4 are 1/2" larger diameter and the differential is beefier than the parts in a 8-1/4. If you put the 8-1/4 parts in an 8-3/4 carrier (which would be extremely difficult and costly as already pointed out) you would weaken it to the level of the 8-1/4 so the ONLY thing you'd gain is it'd look like an 8-3/4 rear