Hi Cameron,
The 17 or 18 inch wheels will give you the right choices.
I've found the Toyo Proxes R888 to be a brilliant track tyre, and of course is still a legal road tyre.
http://www.toyo.com.au/Proxes%20R888%20Page.htm
This means of course that you can prepare the Cobra, and drive to the meeting, compete , and drive home again, which is something I've enjoyed now for years.
Then there is the option ( in 18 inch ) of something like the Michelin SLICKS , ( Carrera Cup Tyres )
This would put you in the "Race Car " category at the Nationals....so then you could Join Scotty / Matt and Steph in kicking my butt in that group if you like.
Regards,
Warren.