The 1999 911 Carrera is in AS. BTW, it's always better to ask these questions of the rule book rather than an internet forum because classifications can change and "internet wisdom" is often worth what you pay for it. Your question is quickly answered by referring to Appendix A of the current rule book which you can download here: http://www.scca.com/pages/solo-cars-and-rules
"no ST class for any Porsche" is false. Not sure if there is a ST class for your specific car, but again Appendix A of the rule book linked above will tell you that.