don't remember the rule on solid bushings because I don't have solid bushing. You'd have to scour through the rule book I listed above, wait for another person to answer on here, or ask on the scca boards.
CST class is found in our local region site's supplementals - should be on
California Sports Car Club - Solo - SCCA.
It has very simple requirements:
1) The car has to be "capable of being registered" in California
2) The car has tires that are 140+ treadwear