I'd hang on to the diff and go for the 6 speed box Claudio, that will sort it.

Please trust me when I say 2.93 ratio will PISS YOU OFF... it's not worth a few rpms lower on the motorway.

Edit: Standard m52b25 (323i) and m52b28 (328i) diffs are 2.93:1 very, very long gears. The 3.15 out of the m50b25 (325i) or a 3.0 M3 diff might be a good compromise but I wouldn't bother to be honest. Large case 3.23 from an evo is a good option but You'll have to swap out too much for it to be worth it I'd say.