So the outside of the drum is stamped something like "max dia. 11.08"? If it is then they're 11x2.5" drums. They do make 10" finned drums, but if they measure 11" on the inside then they're 11's.
The 11" drums came on plenty of cars, including cars with 8.25" rears, but they're not stock to any A-body. They share the same offset and everything as the BBP 10x2.5" drums though, so they'll work just fine, they'll bolt right on in place of the 10x2.5" brakes for an easy brake upgrade. It's possible someone just upgraded the brakes on that 8.25", but I'd want to make sure that's actually an A-body 8.25". Also, if you're not running disks up front though you'll be way rear biased on your braking, you'll probably need an adjustable proportioner in the system.
I run 11x2.5" drums on my Challenger, Duster, and have them set up on my EL5 GT (not on the road yet). But I also run 11.75" front disks off the later B/R bodies on all of my cars as well to keep the front/rear brake bias where it needs to be. You also need to run 15" rims with the 11" drums.
