I've used the SmartStrings setup. It's very nice.THe wider rear track of the M3 does not matter. Remember a 911 has a slightly wider rear track too. The key is that the strings be set up such that they're parallel to the longitudinal centerline of the chassis.
For those who don't know, the SmartStrings are for setting toe. You set camber and caster with the SmartCamber tool.
Instructions
http://www.smartracingproducts.com/instructions/index.htm