I ran a set of 6 1/2" backspace wheels with my 8.8 like yours and they fit under the fender (almost). Your Mustang Fox Body rearend is between 58 and 61" wide and what B&B PROVIDES NOW is a 52" wide axel.
Here is a link for a narrowed 9" housing, axels, bearings, and retainers:
http://cgi.ebay.com/ebaymotors/9-For...QQcmdZViewItem
This would bolt into the same place as your 8.8 but you would need to get some brakes and a third member.
Clois