This weekend Capt_Furious and I installed NA Miata seats into his early 260z. I know there are other Miata seat posts in this thread, but hey, why not add more of the same
Picked up a pair of NA Miata seats with rails for $100. No major tears - just some usual wear on the driver's side bolster, but no holes or major tears. These have speakers in the headrests, so the wires were cut hanging out of the back. They were dirty and got a decent cleaning from a wet-vac (I needed to rug-doctor the carpets at home, so... this worked out).
Miata seats rails. The spacing between stock Z rails and stock Miata rails is about 3/4" difference.
The guide pins and mount tabs are riveted in, so they had to be drilled out and they pop right off. The mount tabs on the other side were cut off.
We used the guide pin hole as the reference point and drilled new holes in the back of each rail. M8x1.25 45mm long bolts were welded in so they wouldn't spin. These were regular hex head bolts and cleared internal rail guts just fine.
When we removed the driver's side seat, we found that the mount tabs were cracked/torn. The rear mount plate fully separated at the spot weld and had a huge crack, so I welded in 1/8" plates to fix this. Ugly but functional.
Drilled new holes off a cardboard template I made and the seats went in like butter. The seat height is taller than stock and the headrests are lower than Z head rests, despite the seats looking to be of the same height.