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I also have 265/35/18 front & 305/30/19 tires (Michelin PS2). The front wheels are 18x9.5 59mm offset, the rear are 19x11 65mm offset. My car is lowered with the stock bolts. The tires don't stick out or rub, and no spacers are used.
Since your car is not a ZO6, your stock tire dia is ~.5" taller, so you will have more fender gap using the tires mentioned in your post. If you don't want to lower the car, consider using 245/40/18 & 285/35/19 tires (stock C6 sizes), as they are very close to your stock tire dia.