I ran my first auto cross this weekend on street tires and noticed the obvious trend...people with racing tires have an advantage.

I have a few questions regarding the upgrade to racing tires. I'd like to keep my Civic in the HS class, so hopefully somebody can help. The rules in the SCCA rulebook are a little confusing to me. http://www.scca.org/_filelibrary/File/2005SoloRules.pdf
Section 13.3 E seems to say that race tires are not allowed. But I know that's not true, so I must be misinterpreting the rule. The other rules seem to state that I can get any type of rims as long as they are the same dimensions as the stock. And I can get about any tire as long as it can fit on the stock rim. Is that correct? If so...how do I find out the widest tire my stock rims can fit? The stock tires are 185/65/15. I'm just trying to make sure I don't violate any rules.