8 3/4 swap questions...

If it's a true BBP 7 1/4 rear end, it'll be out of a '73 and up car and was transplanted in at some point in the car's past.
It will also have the 10''X 2 1/2'' brakes and backing plates on it already, and you can reuse them on your 8 3/4.
It is also wider than a pre '73 SBP a-body 7 1/4 rear end, i'm not sure by how much, but it is wider.
You will need shock plates and u-bolts for the 8 3/4 size which has 3'' axle tubes.
You will probably need the brake hose and lines for an 8 3/4, plus your drive shaft will need to be shortened.
I would suggest that you get ahold of Dr.Diff for the axles etc. that you need, he is great to work with and his prices and service are very good.
This all being said, the a-body 8 3/4 is narrower than a BBP 7 1/4, but when you put BBP axles in the 8 3/4, it in effect widens it so your wheels will be close to where you are now as far as clearance.
I hope this helps...........