8.25 vs. 8.75?

I believe the axle shafts on the 8-3/4 are stronger, and the bearings don't ride directly on the axle in that rear. The 8-1/4 is a c-clip rear end that the bearings ride on the axle shafts if i'm not mistaken. On the plus side of the 8-1/4, a lot of light duty trucks used them.
Since you have it, I would certainly switch to the 8-3/4 and be done with it. Are you sure the rear you have isn't the 7-1/4 under there now ? The covers on the back are very different looking.