It can be done,,, they sell the parts .... but it wont come cheap and keep in mind the that special Saleen exhaust is a big part of "the look" (even more money)
He's right, the rear bumper kit is $800 and the exhaust is $1000. Plus the rear bumper says SALEEN across the back. It may look like a parts car without the rest of the kit. http://forums.stangnet.com/attachment.php?attachmentid=41166&d=1135285550
For $1800 I'd personally put it towards a supercharger or a new set of rims.