I can't believe that there isn't one Mustang in this game. There doesn't even seem to be any mods, not even bad ones. A Saleen mustang or a GT4 something to run against the Camaro. A little trans am action at Sebring.
There is a modern GT500 somewhere, I think it was converted from rF1, so it was probably a Forza conversion to begin with...
I've had the pleasure of Auto-crossing several of my friends Mustang's over the years. It would be great to have more cars represented within the GT4 class imo, and the Mustang would be a fine addition. The new Mustang's have come a long way in terms of handling, still a bit heavy but, tons of fun.
I know the FVR V8 Supercars guys are working on the 2019 version which will include the Mustang, not sure how far away that is though.