I would just get OEM M3 E90 parts to do your conversion. Once you replace the radiator support, all the dimensions will work out fine. Just make sure to change the fenders, sideskirts, and windshield wiper fluid container. You will need E92 headlights or E90 M3 headlights (they are basically the same) and new headlight brackets. There are several other odds and ends, but you get the point.
Don't bother trying to do E92 conversion...you will need custom parts here for sure and the cost will be the same in the end.