If yo're going to run SCCA events - stock class is pretty much limited to very minor mods. Running "street prepared" gives you alot more leeway but also puts you up against major $$$.
See the link below:
http://www.scca.org/contentpage.aspx?content=61
In the middle of the page are downloadable rules but more importantly there's also the class descriptions.
Where are you located?
Good luck.