Well the stock specs on the 18X8's is 5.5" BS with a +25mm Offset and for the 17X7.5 is 5 3/8" BS with a +30mm offset.
I am currently running a 17X8 American Racing Mojave Teflon wheel with 4.5" BS and zero offset with stock BFG's and they don't stick out to far at all. I have the leveling kit as well and no rubbing but then again you are going a little larger but I don't think you should have a problem as long as you stay away from anything less than 4.5" BS and NO Negative Offset but being that you are going with a 9" wide wheel I would get as close to 5"BS as possible. And yes the bolt pattern is 6X5.5
I am currently running a 17X8 American Racing Mojave Teflon wheel with 4.5" BS and zero offset with stock BFG's and they don't stick out to far at all. I have the leveling kit as well and no rubbing but then again you are going a little larger but I don't think you should have a problem as long as you stay away from anything less than 4.5" BS and NO Negative Offset but being that you are going with a 9" wide wheel I would get as close to 5"BS as possible. And yes the bolt pattern is 6X5.5