My friend runs the 16" (? I think that's the size) Ultra Shield Spec Miata seat in his Spec Miata with a 17" Kirkey Pro Drag passenger seat.
The Ultra Shield Spec Miata seat is very similar to the Kirkey Pro Drag.
Neither seat has the large wrap around wings the Intermediate Road Race seats do. Those can get in the way if they are not your size.
We lined up a bunch of seats, in different sizes and styles and tried them out. We decided a 16" was a good compromise on width and we liked the Pro Drag/Spec Miata style better. So I swapped the 16" Intermediate for a Pro Drag and we've been happy ever since.
We have had a variety of different sized drivers both tall and wide and the seat has been very versatile.
I am sure you can fit a 17" aluminum seat in a Miata since my friend has one.
I mounted my seat on sliders with angle iron adapters. The seat bolts in through the sides. The angle iron can be shimmed front and rear to adjust the layback which can be used to give tall guys more room by reclining them.
Additionally, Hans recommends a 20 degree layback too. It seems to be comfortable and we did not tilt the seat back any further.
Kirkey makes a bracket which I think allows for adjustable layback too.
Calpolo, depending on where your located, maybe you should try and swing by a Miata prep shop with the car.
Troy
#35 LRE
1973 Datsun 240Z