As long as the wheels are within a certain % of stock size you're golden
And R compound tires (think DOT racing tires) are allowed in SCCA's stock classes anyway
About the only thing you can do suspension-wise is fresh dampers. Poly bushings put you into a touring class at least (more freedom though, touring lets you do pretty much anything suspension wise).
My Fiero (when I can get out with it) runs in C Street Prepared because of the loss of the cat and the addition of the Truleo combo