Best search for posts by myself on the subject, its been covered a few times now
Here's the basics
You need the S13 diff, 2 left hand silvia shafts and one S13 inner 6 bolt CV joint housing and related driver side inner shaft....
Fully strip the S13 LSD, and removing the ring gear from it. Throw ring gear, case, shims etc in the bin
Then strip the S12 diff, and rebuild it with the S13 inner diff unit. ie bolt the S12 ring gear to the S13 unit.
Then the shafts need some work.
You use the short S12 shaft on the driver side with the S13 inner joint/inner
On the short side (passenger side) you can use the standard short shaft but you have to remachine so its like the S13 shaft.
