The car is in E Stock (ES). We follow the SCCA Rules. A current copy of the Rules are here - http://cms.scca.com/documents/Solo_Rules/2009_Solo_Rules.pdf
Changing the seats will move you into Street Prepared, specifically CSP. I think stripping the rest of the interior takes you into a Prepared category, but I am not sure.
You are samrt to tkae into account the effec of the modificaitons on your class. A lot of people don't do that before they start autocrossing.
We have a school coming up 4-25 and an autocross on 4-26. Come join us when you can.