If you are just going to add a IRS to the Mach 1 I would say go with a '01 Cobra and do some stuff from there. One of the best things about the Mach 1 is that it is coming with the live axle which is why it is pulling better times then most Cobra's minus the '03
As for the Roush, they look nice and and all, but everyone I have seen run at the track is a pig.