A 99 - 04 Mustang Cobra IRS swap is pretty much a bolt-in. Why bother with the T-Bird IRS? They are 5-lug, but 4.25" pattern rather than 4.5" like your car.
There are always Cobra owners out there drag racing their cars, looking to swap for a solid axle. Check out some of the Mustang sites for people looking to trade.
I would go with a stang IRS rearend. The upper control-arm looks
like it would be more stable. Than the single upper control arm
from a SC BIRD/XR7 IRS rear end.
Here is some pictures of the Cobra rear subfram complete,less
the aluminum pumpkin.Tese picture came from a ebay sale,forgot
the address,but it still may be for sale.
I figured that was the case. Lots of old Birds around for cheap these days.
I have seen cases where Cobra owners have traded straight across, or asking for very little money, to go with a stick axle. Keep an eye out in the classifieds www.corral.net and www.stangnet.com.