The center box can be two round boxes with approx 400 in size ( this replaces the cat) , then that joins to a odd 200 box in size (this has to have the torque balancers before and after the box in place, which then joins to a offset freeflow box in the rear(approx size of 600 or 800)
This is just roughly the measurements i remember, When im back in the office. i will put up some pics at of how the install should look etc.
Please , please dont just rush off and install the exhaust as you dont want to regret the choice and you dont want to be spending more money after a wrong install.
E36M exhaust is very delicate and has to be built according to spec otherwise car is going to be just a normal Beemer.
Also bear in mind 57mm tubing used up front which then goes into 63mm tubing at the rear.