The Chevy guys used to have it easy...if you wanted affordable IRS, go find a stick shift Corvette (the automatics had the weak Dana 44 midsection). But if you wanted IRS on your Shelby replica, you were stuck with a Jag setup, or the newer Cobra setup.but for those unaware, go to the truck aisle of your Pick'n'Pull, and check out the Ford Explorer or Expedition (which uses a 9.75" center section). the Explorer seems to have a 5 on 4.5" bolt pattern, 8.8 inch midsection, and the codes represent:41=3.27:1 ratio, no limited slip43=4.30:1, NLS45=3.55:1D4=3.73:1 limited slip (LSD's tend to lose their clutches after 30,000 miles)44=3.73:1 NLs and Interactive Vehicle Dynamics installed.You may find these cheaper than a cobra IRS for your Fox or first gen, but there still may be an issue of what cradle to use for mounting all those loose parts. Still, this may give you a starting point.
Just because you can't do it...doesn't mean its impossible