8.25 vs. 8.75?

I would say they are pretty close performance wise, but my question would be regarding the bolt pattern. All 8 1/4" are large bolt pattern, so is your front assembly large bolt also? That would explain why the 8 1/4" is in there instead of the 8 3/4". If the front is small bolt and the 8 3/4" is small bolt also, I would change to all the same to avoid having to carry 2 spare's. If the 8 3/4" is large bolt, then the axles have been either redrilled, resized or are aftermarket unless it is an early B body rear. A little more info would be helpful to determine the best combination for your set-up, but all things equal, the 8 3/4" is a superior rearend for a couple different reasons. First, the center section is easily interchangeable, and the larger ring gear is stronger as is the pinion shaft from the 742 and 489, and the variety of different centers and gears and they're availability is to me more desireable!! JMO, Geof