A 225/40/18 will fit with no rubbing which is 25.1" tall, so height shouldn't be an issue. Shoot a PM to Josh (he's on the Top 25 list with the LS1 FC in OR) as he has a 245 or 255 up front with rolled fenders, so he might know more as far as the back-spacing or height right at the lip.
http://www.miata.net/garage/tirecalc.html