The T-bird IRS is too long for the mustang, but you might want to check the Mustang Cobra's with IRS. I think they are the right length for your mustang and come with the mounting cage already set up. All you have to do is bolt it in with 4 bolts, I think.
Check the classified ads on
www.corral.net for Mustang parts. The Cobra IRS comes up quite often from guys changing over to straight axles.
The T-bird hubs can be redrilled for the Mustang 5 x 4.5", but the Cobra IRS is already the right bolt pattern. So, you can either have the T-bird unit redrilled for around $75 including shipping, or buy the Cobra spindles for around $85 for the pair.