Will this setup work on a fully stock Z (not lowered) without any issues in the rear or front? Thanks.

 

yes it should unless the offsets are in the negatives.

 

Use a offset calculator to compare your current wheels to the new wheels. With the new dimensions, check the car with a tape. Do you clear the suspension, brakes calipers and fenders? Reference: http://www.1010tires.com/WheelOffsetCalculator.asp

 

Do you guys think a 275/35/18 or 275/40/18 tire would look better at the rear (stock height). I am asking this because I have no plans on lowering my Z.

 

I would do a 40.

 

you won't have any issues. I ran this exact setup and I was lowered on eibach prokits

 

I run rota torque 18x9.5 with 275/40r 255/40f on sportlines with a little room to spare in clearance.

 

I have Rota Torque 18x9.5 too and I run 295/35 for the rear and have no issues. I also run 255/40 for the front for right now and plan on running 275/35 for the front soon. I think they'll fit fine. I'm on Tein S Springs also.