Go with the 03-04. Sick power. Exhaust, intake, pulley, and tune will get around 500-550hp. I'd say for roadcourses leave the IRS. But for drag racing get a solid rear axle. Get the Mach 1 rear axle cause it has the 8.8 rear end and the same size rear brakes as the Cobra. Or a GT rear end. It's still the 8.8 but with smaller brakes. Or just go all out and get the Ford 9" rear end.