The seats are from http://www.jkcomposites.com/7%27s-dimensions-page.html
These are the same seats first350 used is his built, so check out his excellent build log.
I like the seats and they do a range of widths, I cannot remember the ones I selected I think it was the next to widest size. They also do a full seat base option, so you just bolt it to a runner you do not have to use the flange shown in the pictures. I used a Corbeau(sp) universal seat base that worked out perfectly. If anything the seats are a little low, I might make some spacers to raise them an inch or 2, for the road or track I think it would be ok, but for AutoX I think I will lose site of the cones too quickly. The driver seat does totally block the hand brake mount, so at the moment I have no hand brake. It did use the stock Miata unit bolted to the side of the tunnel. With these seats you really need a top of the tunnel setup.
My only issue with the seat is they are missing the 5th hole for the 5/6 point harness. It was in the original order, I even put a note in the PayPal payment. But the seats arrived without the 5th hole. The manufacture was helpful at first and realized there mistake, offered to fix the situation. Sending the seats back to the UK is not really a practical option. If I can build a car I can out a hole is a seat. But my requests for detail pictures and some instructions has fallen on deaf ears. So I like the seats, but I would be very cautious dealing with them again.