Long tube headers, not shorties, motorsport cats (100-cell), low restriction cat-back, cold-air intakes and a tune - without opening the engine to go higher in compression and use E85 exclusively, not a lot else to do.
Take a peek at the Z1 400-hp thread as well - they have modified the induction side to optimise gas-flow on the intake side with Stillen intakes.
Search is your friend - I'm surprised you have not found all this already.
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