That part in the diagram you linked to is the catalytic converter. As you can see, the 2 to 1 decrease happens just prior to the cat, so anything you do from there back probably isn't going to make too much difference assuming you intend to keep the stock cat in place. Theoretically you should be able to replace all of that with the parts from a 328, but you might need everything from the headers back and that would probably be very expensive. The sedan mufflers will fit on the ti, assuming you have a creative muffler guy. See the diagram below for reference.
http://bmwfans.info/parts/catalog/E3..._lambda_probe/ BTW, your rattle is probably related to part number 8 and its associated pieces in your diagram. Those tend to deteriorate and allow just enough movement of the pipes there to rattle against the remnants.