hey charlie,
i'm almost positve you'd be in the SM (Street-Mod) class. The cat back and springs im almost positive would put you in that class, rather than any stock class. It is possible you could run STS, basically modded cars on street tires. So, either one of those (if i were you i'd hope for STS!)
Dont worry about being competitive yet, just do like i do and go to have fun!
I think the next race thats anywhere close is the 19th of august. I'll post the link i have with the shedule when i get home.
Mark