The NSSCC is a member of the Midwest Council of Sports Car Clubs (MCSCC), and should therefore be using their rules, which can be found here...
Midwestern Council - High Speed Autocross car classifications.
All SRT-4's, including the ACR, are classed in A-stock for MCSCC clubs doing road-racing events such as high-speed autocross (basically for everything except parking lot autocrosses aka SoloII).
Here's the "mod list" of what would take you out of stock class and put you in a different class, such as A-Prepared, A-Modified, or A-Race:
http://autocross.mcscc.org/competition/mcpoints.pdf
And yes, older Corvettes and 1990's Mustangs are in this class, but the SRT-4 holds its own and the ACR does quite well against them (given equal tires and drivers of course).