for what it's worth...
I read through this epically long thread and saw a few mentions of using the 99-04 Cobra IRS... and then that it wouldn't work with the Jag stuff anyways. Well, the 99-04 Mustang rear ends were a good deal wider than the one used in the Kugel Komponents kit (79-93 Mustangs). The frame rails are the same, and the same width from 79-04, but it's a good amount of work to make that IRS "fit" in a Fox body... good luck getting it in an adapted Focus.
2000 Ranger XLT 2.5L/M5OD-R1, 1984 Mustang SVO (restore & sell), 1984 LTD Brougham wagon