Personally I would never swap an 8 1/4 (if that is what you have) for a 7 1/4.
If your rear end is an 8 1/4 you will also need a drive shaft.
If their inventories are current these places may have a sbp rear end.
Charlie's (76 miles from your zip) is showing two sbp rear ends with 10" brakes.
http://www.charliesusedautoparts.com/
Salvage Hunter (42 miles from your zip) is showing one sbp rear ends with 9" brakes.
http://www.salvagehunter.net/
JP's Muscle Cars (76 miles from your zip) is showing one sbp rear ends with 9" brakes.
http://jpmusclecars.3dcartstores.com/
New Dixie Used Auto Parts Atlanta, Ga 1-404-873-2400 (47 miles from your zip) is showing one sbp rear ends with 9" brakes.