I'm running 18x9 and 18x10 with backspacing for a 94-04 mustang. I needed to use 1/2" spacers in the front and 1/4" in the rear. You'll likely need to tweak these slightly for the best fit. Tires are 245/40 and 295/40.
With an 18X9 the spacer is needed for upper ball joint clearance. Spacers on the rear are to center the tire in the wheel house.