I know that the s13 is a little bit wider (on the outside) than the silvia. But not too sure about the inside. However, when i was modding an s13 dash to fit the s12, I had to cut about an inch off each end of the dash to make it fit
. So the s13 cage will probably be a bit too big for an s12. Hope that helps a bit
.
Regards,
Ian.