I'm considering either a thunderbird or cobra irs set up and possibly doing a mustang sub frame. Lots of fab work, I know. The width of the mustang rotor to rotor is narrower than the ranger. Which I will address with negative offset wheels and possibly spacers. I was just hoping to be able to avoid fabricating. Otherwise, I'm going to go with a mustang 8.8 with disc brakes, lower the truck, and add a quick steer. I'm stuck with possibilities...
The thunderbird IRS is a different bolt pattern than your truck, so you'd have to convert the hubs to mustang cobra pieces, or run dual pattern wheels or adapters.
The Cobra IRS is supposed to be a better overall suspension design than the MN12 in the thunderbird/Mark8, and it's the same bolt pattern as a Ranger so it would eliminate a bit of work, but it's going to be more expensive up front.
Both IRS assemblies will be a few inches wider than your solid axle, so you'll either have to widen the bed, or run narrow rear tires with crazy offset wheels (assuming you'll be lower than stock at that point).
As for the front, from what I can tell from a quick googling, the only Mustang with a narrower track width than the Ranger is a fox body car. The SN95, New Edge and newer cars would all be wider which would cause some issues.
Just some stuff to consider before you start cutting on the frame. It's a big time project that requires a lot of time, money and skill. You'd probably get 85-90% of the performance with far less cost and time just by dropping the truck with standard Ranger parts, getting thick sways, and cramming as much sticky tire under it as you can fit. Just depends how much time, money and sweat you're willing to invest.