A smaller than normal wheel/tire package will make it easier to keep the hide fire lit.
You'll need:
Complete bushing package from Full Tilt Boogie Racing with cross axis joints.
Diff cooler (I would look at Kenny Brown or Maximum Motorsports)
Rear end you'll want a coil over conversion and here you have a choice to make, Kenny Brown full 9 yards IRS ($$$$$) or Maximum Motorsports just coil over conversion and tie rods ($$$).
Looking at at least 4.30 rear gears and you'll probably want to go to a sequential transmission and carefully chosen gear ratios.
If you want to drift well or competitively you're going to have to deal with the front end even more than the rear, you will need to substantially modify the front end so you can turn about 20 degrees more wheel angle.
You'll very likely have some induced bump steer and rear steer problems that you will need to fight through.
Drifting an 99-04 IRS Cobra is a ludicrously bad idea unless you start with the full house treatment done to it. The 99-04 IRS was an afterthought, shoe horned into a car that wasn't designed with it in mind. There's a lot about it that's not super wonderful. If you go in with stock bushings, stock cross axis joints and the factory damper and spring setup you will absolutely tear it up, probably you'll rip the front ears of the diff center section and then you'll break the rear cover and then you'll get to spend another thousand bucks repairing the damage. Give the IRS to someone that needs it for grip instead of slip and stick a live axle in it with a Maximum Motorsports or Griggs torque arm and panhard bar 3-link setup in the back.
