Your car would be in STP - which is a new class for this year. You could also run the car in CAMC. Most people with Mustangs do not use 20" wheels. There are no really competitive tires available in 255/35/20.
If you can afford it, I'd go down to 19's and run 275/35/19's - one of these tires - http://www.tirerack.com/t...&rearDiameter=17
Hell you could even go down to 18's and run 275/40/18's - http://www.tirerack.com/t...&rearDiameter=17
Also, to be competitive in STP you'll want stiffer springs, better shocks, and stiffer sway bars, but don't let that stop you from getting out there and racing. You can even race the car exactly how you have it now.