You can read the Solo (autox) portion of the Fasttrack news (monthly rules update) here:
http://www.scca.com/assets/13-fastrack-aug-solo.pdf
TLDR: Not enough people were competing in STU so they moved Mustang, Camaro, Genesis, and 350Z to STU in response to people asking for more tire. They also put C5 Corvettes in STU...
Then submit your letter via:
SEB Letter Submission Form with category Street Touring and STU class.