The reason I originally mentioned the 350Z is because I am installing a 350Z engine in my S14 and wanted the same diff ratio that comes in the 350Z. The Q45 uses the same gears but will bolt into the S14 unlike the Z33 diff that takes a bunch of fabrication. I found the web site that explains the 350Z to S14 differential swap. This should clear things up I hope. The end result with the Q45 diff is you get the desired ratio in a housing that bolts in.
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350z Differential Install into a 240sx Dennis Mertzanis