18" Wheel help

Would a 255-40 rear tire work? I was looking at the Nitto NT555g2 but they don't have a 255-40 for the rear.
I was thinking 245-40 for the front.

A 255/40/18 should work fine, again, maybe a small spacer needed.

A 245/40/18 should work in the front, that would about the maximum for fender clearance with that set of rims