[ausie]

There is a lot of info on this subject I have seen in many other sites as well as in here. svtperformance, modularfords, allfordmustangs, etc.... If you do not get an answer to your question in here, try the other forums. The more information and advice you can get will aid you in making the right decision. Expect to have pro and con opinions made as some like the IRS and others do not. I may be biased towards keeping the IRS than opting to swap it for an SLA (solid live axle I believe is the correct term but others use SRA for solid rear axle ).
The axle assembly from 2000 up to 2004 GT should fit since the IRS is an assembly that uses the same connecting points as the Mustang GT with the exception of the rear mounting points as well as how the springs are mounted. Not sure if the 2005 axle assembly will fit or not. You may be able to use the differential but since the SLA of the GT may not use 31 spline axle shafts you may need to get new ones if you keep the traction loc differential from the IRS. Not sure if there is anything else that could be reused, you many need the pinion flange assuming that the drive shaft will not be required to change (you can buy one new online for around $20 to $25, at ford it would cost you $52). If you are serious about drag racing, why use an 8.8 rear from a GT? Get a Ford 9 inch rear end which would be much stronger and able to handle the abuse, maybe overkill and expensive (if you go this route, you will need more than just the rear end assembly). If you opt for an 8.8 from a GT, have the axle tubes checked for alignment then have the tubes welded to the differential case (they are a press fit assembly). Reinforcement can be added to the SLA to beef it up. The nodular iron used in the SLA is not bulletproof either. It can crack, spit or be damaged by hard launches over time. Just a point, swapping to a SLA will not resolve wheel hop issue. Both the Shelby GT500 and the Mustang GT have had issue with wheel hop at the drag strip. The only difference is the cast iron axle tubes and differential housing will hold up much longer than the aluminum differential carrier and CV joints in the half shafts of the IRS cars. Also, there are ways to secure the SLA suspension that will eliminate the wheel hop which cannot be applied to the IRS cars.

I have read many threads on this topic else where. Many that have made the swap wanted to go back the the IRS. I have had two IRS cars, a 2001 and now the 2004. My 01 had issues with wheel hop. I could not do a simple burnout without risk of damage to the half shafts or differential components and it got worse when the roads were wet. Needless to say, I never drag raced the 01 so how it would have performed at the track I will never know. As for the 04. Wheel hop is not an issue. I have made at least 35 passes at the drag strip without a hiccup, burnouts were smooth and never jarring. If I ever got the rear end to hop it was just a matter of letting off the gas. I may have had wheel hop a few times when driving in the rain but not often at the track. Then again my attack on launch is mild and I use the clutch and gas to regulate wheel spin. The factory clutch did not last very long.