Morris,
You'll like the Miata Seats. And you won't find your a__ melting into pleather seat covers in the Texas Sun anymore. Speakers in the headrests are a treat. I added a set of Bass Blocker capacitors, like $7.50 a pair over internet, Google Bass Blocker. Really help to keep from overdriving speakers in the headrests. A couple of different ways to mount seats in the car. I used a 1" piece of bar stock under the back seat mounts and just cut off front locating studs. Can reuse front inside stock seat mounting location plus 3 new holes needed.
Use backing washers or steel plate under seat floor.