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Stock wheel dimensions for a C5 are 17x8.5 front, 18x9.5 rear. I don't know the offsets of those.
The Z06 wheels are 17x9.5 front, 54mm offset and 18x10.5 rear, 58mm offset.
The bolt pattern for all C5's is 5x120.65mm (or 5x4.75")
You can certainly run an 18"wheel in the front...the width, offset and tire size will determine if you rub or not. With only an 8" wheel, I doubt that would happen.