Yup it's the perky butt.
I kinda like it myself. FFR had to make some compensations to use the stang donor parts. If they had thought to make the rear a little wider you could use a wider tire without having it stick out from the fender lip. You can also do two things......first using the original FFR body you can drop the butt 1-2 inches. This is know as the Fun Ford drop butt mod. This gives the car a more original stance and look but you still have the narrow hips. Do a search on the FF forum
www.ffcobra.com for the "drop butt modification". There are before and after pics to see. Secondly Mike Mac from Tri-States Motorsport has an aftermarket body for the FFR. Wider more original rear= wider rubber. Finished underhood and trunk. And READ: NO SEAMS. For $3295 take a look he's a vendor here. A stunning body for sure! As FFR will only credit you $800 if you delete the body some guys are selling them on the forum and ebay and getting $2000-2500 and buying Mike's body. So there are options......G.