Ridetech is also releasing an S550 based irs for foxes as well. It’s utilizing 2015-2023 Mustang driveline/brake components.
Looks like it will be transformative for handling and it keeps stock fox rear track width.
Independent Rear Suspension System | 1979-1993 Mustang
You need to supply the s550 diff, hubs and buy the shortened rear s550 axels from them. Will be an expensive upgrade but this may be offset but a significant reduction in labour and installation costs.