It's difficult to find a replacement seat that fits properly in a Spitfire/GT6. The Miata seat, for all its praises, simply does not fit well. It sits higher in the cockpit (which is already very small) and restricts leg and foot room. It caused my leg to wedge uncomfortably against the tunnel cover. It also restricted foot room on the accelerator pedal, making long trips very uncomfortable. I went to the extreme of cutting the rear of the Miata seat rail and rewelding it back on to shorten the rail and make more room, and it still wasn't enough.
Some folks love them, most of whom are under 6' tall.
Please carefully consider restoring your stock Spit seats. It's more expensive, but in the long run, far better than wedging a foreign seat in your Spit. The original seats, when carefully restored, are quite comfy. And of course, they fit the car perfectly!