Switching from 8-1/4" to 8-3/4" rear ?

Also worth mentioning is the rear axle width. An A-body 8 3/4 with BBP axles and brakes is about an inch wider overall than an A-body 8.25. So, your backspace will change by about a half inch per side. If you're running tires that are perfectly centered and maxed out for width already you may have an issue with not having enough backspace with the BBP 8 3/4.

So take a look at your tire to quarter clearance and see if you have a half inch or so to give up. You'll need about a half inch of clearance to keep the tires from rubbing on the quarters when you're done.