The Lincoln Versailles came with a 9in rear w/ discs that will fit a Mustang. I personally have one out of a 79. It's a little narrower than a standard Mustang axle so you can also fit fatter tires =) You'll more than likely have to swap the thirdmember to get posi... check my webpage out for some pics and stuff.
http://members.xoom.com/Jefrezicus/
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1967 Fastback, DSS Racing shortblock, Lunati Cam, TFS Heads, Tremec TKO, basically the works... =)