8.5" @ ET35....I think it's going to rub the coilover and you'll need spacers. I'm running 7.5" ET35 and I can barely get a finger between the tire and the coilover.
245/35-18 is the correct size. 245/40-18 is a bit big and will make rubbing worse.
Why go 18" for track/autox? I'd drop down to a lightweight 16"....or maybe 17" depending on tire choices.