Not sure if people know about these, but there are a few documents outside the actual rulebook that help with new folks classing their cars (and even deciding where you might want to go with your mods).
I don't think I can link direct to the docs thanks to the way the SCCA web site DOESN'T work, but they are easy to find. Go to
http://www.scca.com and then click "Solo" at the top. Then click "Solo Cars and Rules" on the left. You can find the rulebook there, but there are other documents that help if for nothing else than being a faster way to find things like stock class cars by make.
These documents are occasionally updated and the date of the last update is usually beside them, which is kind of nice.
--Donnie