I got about 95% of the way through the install of my Sparco Speed 2 seat and decided that the seat was too narrow for me and that it was going to make getting into and out of the car difficult. I had a few issues during the install: 1) cutting that bar stock was pretty difficult when you're using a battery powered circular saw with battery packs that barely hold a charge any more; 2) drilling the bolt holes in the exact right spot on the bar stock was also more difficult than I had anticipated, I had to make the holes a little oblong to line up with the floor holes; 3) you will need a vice to bend the bar stock to make it match the angle of the floor (you'll see when you look at the stock brackets).
It really was not all that hard to do, it just took me longer than I thought because of the issues I had. I now have a different miata with a sport bar instead of the hard core bar. I'm finding it difficult to get my head far enough below the bar for safety. I've cut out the "shovel" area of foam and really like how the seat feels now. However, I'm still not as low as I'd like. I'm going to have to go back in and remove foam from the top of the seat.
I'd like to find a seat that will let me sit lower, hold me better, and not overly interfere with entry and exit from the car (I drive the car almost daily). I'm not sure what that seat is though... maybe lotus seats or the Sparco Chrono?