Happy to help! The 18x9" ET42 is a great square fitment for the F3X chassis. We have tested 245/40-18 tires with no issues. We have had customers run 255/40-18 tires as well, but in some cases it may require a small spacer to clear your suspension (with possibly a touch of negative camber). This is dependent on tire brand/compound and suspension type.
We would not recommend going with 265/35-18 tires as those are a bit aggressive and require quite a bit of work to fit properly. In addition, a 9" wheel would not properly support a 265 tire. 255 is more than enough for the F3X chassis.
You can run our 18x9" ET30 wheels, but that will require some negative camber up front and possible fender roll in the rear. Going with a 255/35-18 tire would aid in clearance on that fitment. Check out a customer running the 18x9" ET30 with 255/35 tires
here.
If you have any further questions, feel free to PM or shoot us an email
-Matt