So i just got a set of 18" black with a crome lip z06 wheels. the back spacing seem to be 7.5" off the the mounting flange my question is what size spacers do i need?

if your rims are 18x9.5 then you should be fine with 2" spacers cause thatw what im running with my Zo6 rims aswell with many others on here.
Hope this helps

thanks i just wanted to make sure.

All depends on the offset

what if i got 18x8.5 in front and 18x9.5 in the rear what spacers would i need?

well to be honest im not sure about the 18x8.5,but i would still assume the 2 inch spacers.
What i did was find people with the same year as mine that were running the same size rims i was gonna be running and asked them,plus i read alot of diffrent post on here from others and the majority of people are running 2 inch spacers with the 18x9.5.
If this doesnt help just look around in other threads like i did an hopefully you will find your answer.

i have 18/10 all the way around. 2.25 front and 2.5 rear fits and looks good.