Here is some info about swapping over an S13 rear subfrme. The S14 is similar to the same process. R32 is a little different I think but same concept.
http://s12silvia.com/community/index...s-and-options/
As for transmission.
The RB20 trans fits with only massaging to make room for the starter. No big deal. The RB25 from my experience and Dan's (Canadian who has an RB25 S12), you have to do some work to get it in place. The RB26 is worse for it has that "tumor" on the side that has a smaller driveshaft to connect to the front diff. for the 4wd setup it comes with. For that it would be quite a bit of work. May be worth id but it would be on your driver's side, which would be a pain I reckon with seat placement.