finding a 8.25 rear for an A Body

Hello new to the A body forum and world,, found a nice 68 Barracuda, slant powered, plan to keep it that way, has the small 7.25 rear,, would like to go up to a 8.25,,, question is where would i find an 8.25 rear dif assembly that fits an A body,, lots of them in the local pull a parts in old D-150's,, but how would one subtract the extra width between the drums,,

thanks