if get M3 diff ,will need diff flanges from any other medium case diff but non M3 ,all 3 of them. 328i diff has 2.93 ratio so its not good if U looking for more sharp acceleration . To fit medium case if U have now small one,need side shafts and prop shaft .what engine was in car before conversion? 320i ? so ratio should be 3.45 or 3.88 .Should be best option U could have with engine U have now.