As far as upgrade go to still play in "H" stock, you can do the following:
Air filter (not a CAI)
Cat back exhaust
Wheels - You can run any wheel that is the same size (width/diam./offset) as what was available on you model car. If you don't have a SVT you could not use those wheels.
Tires - street legal tires...."R" comps should be allowed in your region since they are DOT approved.
For more details to the rules, Check out www.SCCA.com
Click on the SOLO tab and check out page 62 of the rule book.