Narrowing an 8 1/4" rear end, how is it done?

63” wide would make it an 8.25 out of a ‘71-‘74 B-body. Even for a Barracuda that will be too wide.

I run a ‘68-70 B-body 8 3/4 in my Duster, it’s 60 1/8” wide and can be dealt with using positive offset wheels. I run 18x10’s with a 7” backspace with a 1/2” offset spring kit from DoctorDiff with the perches at 42”.

I would pass on the 63” 8.25”, by the time you narrow that and get new axles you could have done something else. As @Professor Fate said there are more modern 8.25” options. There’s also the Ford 8.8 out of an Explorer. They have the correct bolt pattern, came with 3.73’s and disk brakes. Many were limited slips too. Your barracuda could use one in its stock width, or you can get two “short side” axles and shorten the long tube side of the 8.8”, which is more straightforward. There’s several threads going on using the 8.8” already here to check out