I would like to add some info to this thread.
NISMO sells a speedo gear mounting kit to move the VSS from the differential to the transmission tail section. It places the VSS in the same spot as it would be in a S13 or S14. The kit comes with everything needed except the speed ring gear.
NISMO Silvia Reinforced Cross 6-Speed Transmission - Speedometer Gear Mounting Kit
No more s15 diffs and signal converters!
Enjoy.