Yea what spider said
I'm sort of surprised it hasn't been done with all the m3 owners looking down upon us beause of our inferior, antiquated suspension. Beats the hell outta leaf springs at least. You'd best be getting a rear clip from an M3 and grafting the floorpan in place, I think people have mentioned the gas tank would get in the way. People have made 2 door wrx's out of 4 door versions so I think it is probably easier than that. Heck M3 owners be re-inforcing the RTAB's anyways so how much harder to weld new ones in place.. still.. it will be expensive either in time or money or both.. Is the trailing arm suspension really THAT bad? M coupes make do with it.. I haven't tracked my car and am finishing motor swap, but I plan on constructing a rear subfame that has adjustable brakets so you don't have to mis-align your rear bushings to adjust alignment.. you can use solid bushings that way.. probably be a good bit heavier than stock subfame but it's in a good location to add weight at least. Definately need more contact patch in the rear, I had my rear tires so worn that I wore through the steel belt on the inside corners and the outside still had some tread.. yea non-adjustable suspension is lame.