One of the coolest Mustangs I've ever read about was a 1968 hardtop that was equipped with a fuel-injected 428CJ, 4-wheel disc brakes, and
INDEPENDENT REAR SUSPENSION, built by Carroll Shelby. It was called the EXP 500 / The Green Hornet.
I'm not sure if there are any current-day kit options for doing a solid-rear axle to IRS swap on old Mustangs, but if there is, it would be one of the most interesting and exciting Mustang builds ever performed. There's always custom fabrication... The EXP 500's IRS was bolted to the leaf spring perches somehow.
I read about it in the August 2006 Hemmings Muscle Machines. Here's the link:
http://www.hemmings.com/mus/stories/2006/09/01/hmn_feature1.htmlI know you guys like to build pro touring cars, and building an IRS-equipped Mustang would be one of the greatest expressions of the pro touring ethos.