With this kit and an aftermarket bumper that provides more clearance, you can likely run most 12" wide wheels without any issues, especially considering most 12" wide wheels have pretty aggressive negative offsets. That being said, I'd probably stick with a 12.50" wide tire to avoid rub on the back side of the front wheel well, which could be an issue with a 13.50" wide tire, despite the trimming you've already done.