I'm suspecting your mustang has the weak 7.5" 6cyl rear? A maverick 8" will work behind a pretty beefy V8 and will fit right in, I'v been told the Maverick 8" is the same rear end they put in the Early V8 Mustangs and Falcons. Currie Or Moser will get you the 9" brand new for about a grand and a half if you want the 9"