Yes it will fit your 328i if you do the dual exhaust conversion, but there will be less mounting points (places for the screws) in the bottom because the 335i has a different underbody than the 328i. It is probably less sturdy but lots of people have done it none the less.
OEM diffuser available at getbmwparts.com
Replica available here:
http://www.e90post.com/forums/showthread.php?t=297247