From what I remember, the 15&1/2 is a newer Rack. The 16" is the older rack for the Mustang II.
Someone does offer the offset bushings to make either one fit. Google "offset mounting bushings" or try this number...UNI 8021520. Those are what are in my notes but I don't remember which shop the info came from. SORRY !
Just keep looking. You'll find the parts.