I had a stock VT V6, I put an exhaust on it. it was straight through 2.5" pipe from the cat back, with a 1 into 2 pipe rear resonator (not sure of the brand/make of the resonator).
I now own a GEN III V8 with a readback twin 2.5" system and sounds very similar to what my V6 sounded like...
for a V6 the VT sounded really good, or so I thought, but I hate the sound of my GEN III.. I blame the balance pipes for making the V8 sound like a six..
you could always have your centre muffler removed / replaced with a straight through piece of pipe. and see how it sounds, I wouldn't recommend removing the rear resonator, although I did that on my VP V6 when I had it, sounded ok, but you get a whistle on hard accelleration, and a bit of a drone..
anything that you do, your not going to make a 6 sound like an 8... only thing thats happening ( IMO ) these days is that people are making there V8's sound like a 6. therefor your task is slightly easier to fool someone of what engine you have..
if you hear a commodore ( V6 ) that you like the sound of, and have the chance to ask them what exhaust they have go for it and ask