Hmmmmm, let see..... you'll need a new...
Front Bumper with new grill and headlight buckets
Driverside and Passengerside fenders
Driverside and Passengerside skirts
Rear Bumper with new tail light assembly
Estimated cost for full kit $2500 minus labor and paint, tack another $2500 for that.
Now, if you have access to the proper tools and moulding materials you might be able to fabricate these items if you have experience with bodywork. I will of course assume that you have none what-so-ever and that you will have to resort to paying an extremely high bill for custom body components being set, transfered to a mold and then squeezed out.
The lights will cost a pretty penny as well and the side exhaust is impossible without tearing up your interior. The quarter window covers mismatched like on the GTr would be rediculous on a street car so your best choice would be to sample those that are already on the market and make a choice between them (MRT, Roush, 3D, SS, Razi, CDC, etc).
I would love to see a complete new rear bumper kit but I would expect it to cost just shy of $1000 with all the wiring and light bars thrown in.
I think making your own is almost out of the question now but congrats if you want to take on that task.
Apparently Ford may make some GTr styled components but dont hurt yourself by holding your breath.
-Dan