Gutting the car is not legal for STS. That would put you in Modified.
Here's a simple run down of stuff that's allowed in the different classes:
Here's our SCCA Solo site: http://www.neoklascca.org/soloschedule.htm
Rates are as follows:
SCCA Members are - first class $18.00 - second class $14.00
Non-members are - first class $23.00 - second class $20.00