The Z3 M44 trailing arms should be the same as 318ti trailing arms. The Z3 2.5/2.8/3.0 trailing arms are wider than the M44 arms. Look for RCV7's build thread. He initially used 2.8L trailing arms and the wheels stuck out too far.
You can cross reference parts using realOEM. That way you can see if the Z3 M44 parts you bought are the same as the ti parts.
http://realoem.com/bmw/partgrp.do?mo...id=47506&hg=33