Go here - http://public.fotki.com/relittle/miata/racingseats/ - for pics of how I mounted my fixed-back seats.
The driver side is on 1/8" steel and bolted to the floor. Passenger side is on the stock slider with a 1/8" adaptor I made.
Here's a shot of the slider and adaptor bolted to the seat.
Questions? Just ask... I included photos of the adaptor I made with measurements.