well being that you have 18's you should run a 265/35/18 or a 275/35/18 you should hae no problem rubbing unless yoru rim has funky backspacing and offset. if the rim is set up for a mustang those should be fine, i have 17's with 275s and droped and i dont rub any.
do the MATH its very easy... take a stock ride height tire, 245/45/17 to figure out the side wall height just say 245 * .45 (the side wall height in this caseis 45% of the width, 245) thats your answer in mm. now you have 18s... so you need to take that into account, your side wall needs to be maller to keep the over all diameter the same. so you need the side wall to be one inch shorter over all, or .5 inch shorter on each top and bottom. you have to convert inches to mm's its 24.5 mm per inch, so you need a side wall that is (245 * .45 ) - (24.5 / 2)
i think if you do the math you will come up with a 265/35/18 will come very close and a 275/35/18 will be a tad to tall, you cant get it perfect just get as close as you can. if you can go wtih 275's cuz they will protect the rim lips better from curbs.