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Looking at Nitto's site I see that the 17" comes in 275/40 and the 18" in 295/35. Those are almost exactly stock Z06 sizes with the front being slightly wider. These sizes will still maintain the needed dimensions for the system.
I just put them on my 03 Z06, limeted usage so far but they feel good. No noise, nice ride, stick very well. They are very sticky and pick everything up on the road, may want to protect the front fender as they will beat them up more than any other tire I have used espicially with the wider 275/40/17. No traction control, abs, etc. issues either.