The wheels on your Z are wider than the wheels on a normal C5 (1", or 1/2"? I can't remember and I'm too lazy to look it up). The tires on your Z are wider than the tires on a normal C5. The Z's tires would certainly fit on a C5s wheel, but it wouldn't be the best fit. The narrower rim will make the sidewall bulge more that it's supposed to, which will effect handling (but only a little).
Will a 265/40/17 fit on a 8.5" rim? Yes and with no issues.
Will a 295/35/18 fit on a 9.5" rim? Yes and with no issues.
They may not be the optimum size rims for those size tires, but IMO everything will be fine. :z: