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You would have to change the half shafts to get the S14 Viscous Diff on. The Viscous Diff is quite inferior compared to the S15 Helical Diff too. Your options are to either get a replacement from Nissan Japan via someone like Perfectrun, or, if you are the drifting type, you could consider a 1.5 or 2 way diff. These can be got from somewhere like Driftworks.
Cost for the diff from Nissan Japan will be over HALF the price from Nissan GB plus shipping. Lead times from Perfectrun are about four to seven working days. Details are in the 'Nissan OEM Parts Suppiers (Japan)' sticky in the Traders section.