Hey,
I don't really know if you would have to modify anything... But, if you do it would more than likely be simple to do... I have seen that kit and I do like it, but I think the intake looks a little weird because it is to the side. If you are having thoughs about getting a different one (Not that you should. Its a good kit) I would check out
www.cervinis.com They have a couple of different Mustang body kits. I kinda like the stalker kit with its dual intake. But for now I am gonna worry about power :s2: Plus, I might just get a Cobra and if I do I think it looks fine how it is.
Here is a page with some good ground effects etc... and some pics of the kit you are looking at. It also has some other different types of kits:
http://body-kits.com/kaminari/mustang.htm