Don't build a car that isn't completely legal for any class, you must build it to fit a class. This might help you with a class. Hmmm, open wheel custom body, not a replica of any production car, 2 litre engine. Sounds like G/GL to me. G is 1501cc to 2000cc, G is for Gas (pump petrol) L is for Lakester, but with 300hp it must surely be turboed (unless it has one of those nifty little Toyota superchargers) so make that G/BGL. B is for blown, i.e. forced induction. If you use nitrous oxide or methanol or nitro (!) you go into F for Fuel class. Get a copy of the 2010 SCTA rule book at the Hot Rod Show this weekend and learn it off by heart.
Build it all so that you can be justifiably proud of the quality of the finished car. It's much better to be on the receiving end of endless compliments about your building skills than having just as many people frowning, shaking their heads and walking off without saying a word to you. Make sure you involve Bob Ellis in periodic inspections of the car during the build so he can steer you in the right direction. It would be a very good idea to start a build diary with plenty of photos on this forum. People will be happy to offer advice and help to keep you on track and guide you through problems that they've already had themselves.
Don't skimp on personal safety equipment either. Don't try to use a second hand harness (it won't be legal anyway). The scrutineers are necessarily red hot on safety issues because the lake is a very long way from any emergency hospital surgical care.
I suspect the movie "World's Fastest Indian" has convinced some people that any old junk just thrown together from whatever's in the shed will be warmly welcomed and allowed to race no matter what. NOT true! If you present a car that, in the opinion of the scrutineers (and nobody else), is not fit to race, you won't race. It's a long way to drive for nothing.