I am not running a SCTA event. If you look at their rules it is really set up so a group of 15 or 20 friends can still hold all the records. Their rulebook is contradictory and stupid .
Basically it says over 199 you need a 6+point cage and a parachute if the car was not originally designed to go over 200.
But as for aero rules there is nothing other than tire specs.
Consider exiting the exhausts at the foremost outer corners of the diffuser to enhance edge vortices. This is how F1's EBDs functioned.
Otherwise I'd look at other concepts, considering its formula libre. Instead of trying to optimise a static design from the get-go, make your parts moveable. The diffuser ceiling, the wing(s). Moveable either for set-up or while in motion. Wing mounts commonly have pivot points, so you might find something off-the-shelf to build from.
If you want static downforce without totally relying on mass, look up the SCCA corvette convertible from a few years back. As a consideration of suction-downforce on a budget.
