That might be early enough to still have the odd tapered axle setup with the threaded flanges or whatever? I think the years you want are 1965-67, someone correct me if I'm wrong.
Again I put a 1967 B-body 8 3/4" in my Duster but those have much wider wheel wells than Barracudas and I still had to get the wheel offset right. Fortunately OTS aftermarket replacement late-model Mustang wheels were pretty much perfect.