bro
Wheel Fitment 101: START HERE
and
18's on a 370z
18 vs 19 doesn't change width/offset choice. you'd need to adjust tire size
also
there's no reason to go 18 instead of 19 unless you're tracking the car. the 370z comes from the damn factory with 19's in several trims
18's look like crap
square on stock suspension is gonna be awkward as hell. The rear needs 10.5 or better, putting that in front requires some adjustments
you need to completely rethink your parameters unless you have an actual reason for demanding a square 18 setup on stock suspension