Looks [expensive] 
Needs S550 Diff and custom axles, brakes, blah blah...
ridetech.com

Needs S550 Diff and custom axles, brakes, blah blah...
Fox Body Mustang IRS Suspension system
Ridetech's IRS suspension system is for 1979-1993 Fox Body Mustangs, is 100% bolt-on, and utilizes 2015-2023 Mustang driveline/brake components.