I also went the inexpensive route on wheels and used a C6 zo6 style wheel in a 18x10.5. I test fit the wheels they fit perfectly and look good for $130 each.
I had the same dilemma and ended up using 305-40-18 in the MT ET SS tire. I wanted the R's but the 305-40-18 is closer to stock size.
If you want to lower the effective gearing slightly, go with the 305/35, if you want a slightly taller effective gear go with the 305/45. Or chose to keep stock gearing and go with the 305/40.
You should be able to come out harder with the 305/35 if you have enough traction.
How about an offset for the wheels? What are you all running with the c6 Z06 rims.
Thanks