Point of information: S14 carpet in an S13
I got some nice S14 carpet for $50 locally. I saw a comment from Estevan many years ago that S14 carpet would work in an S13, so I tried it. The first thing to note is that you HAVE to run S14 seatbelts in this car, and all auto hardware needs to be removed. Otherwise, it's very close....

Here's the wide shot. Check out the center console. The shape of the opening even follows the shifter, random screw holes, etc. Seat rails line up.

A little closer shot of the driver's side. The trunk/hood release lines up great, all seat holes are correct, seat belt holes line up.

Here's where they deviate. The edge against the rear seat comes up 1.5" short. On the S14 they probably added some structure here, as this is a weaker part of the chassis.
Also not shown, with the full interior in, the S13 kick panels leave gaps as the S14 kickpanels are larger.
So there you go.