I'd also suggest a crossover pipe. Without one you've got three cylinders exhausting from each of two pipes. I'm sure you've heard plenty of V-8s without crossover pipes, They sound like crap, and give up a significant amount of performance.
Remember the sound that the 1950's Formula 1 cars with straight sixes made? That's what I'd go for. Note that all of those exhausted into one manifold. A crossover pipe will give pretty much the same effect.
