i autocrossed my miata off and on for 2 years, and ran my 135i for a few events. I'm pretty sure my BMW was in STU as well and I got wrecked by the lighter cars, so I was worried a little bit about lagging behind in the mustang. It's been a few years for me though so I couldn't remember what classing I would be in, think they would get mad if I entered as a rookie again, I won my first two events in R XDIf its your first season of autox dont worry about what class you run, and simply go have fun. And the fact that you are asking what class you are in, this implies that you are new to autox. So go out and have a good time
If you want to be competitive, you are better off taking the wheels and springs off and running in FS . At this point those parts are bumping you into a class that the car is not set up for, and as the saying goes "bringing a knife to a gun fight" as you are under modded for the class to be competitive.