Earl,
What nevermind65 said...
I have done that to the Zephyr. It's great!
First Coast Cobra Club • View topic - Zephyr dirty hands party Saturday 1-24-09
If the pickup points for the present rear are identical to the Mustang's, you're most of the way there; the Mustang IRS also uses the quad shock mounting points, so if those aren't present you'll need to figure out how to add them to the chassis. Also, the rear u-joint is different so that will need to change, but that's pretty easy...
The '99-'04 Mustang IRS was made to "bolt right in" to any Fox Mustang. Yeah, right...
BTW if you decide to do this swap, the '03-'04 differentials and axles are 31 spline, compared with the '99-'02's 28 spline.
Have fun!
Tom