Some updated photos of my Ultra Low Seat Bracket. This mounts directly to the floor using a backer plate. This is meant to drop you the absolute lowest possible in any car. I used this in my MR2 to solve my issues with my height + helmet + big seat. I personally use this with a Recaro Profi XL. I have pictures here with a OMP WRC. It is meant to fit all vertical mounted seats.
There is some revisions coming to this design to make a even better. The kit (whenever it comes out) would includes all the machined parts shown plus backer plates, hardware, and floor spacers (because your floor will never be perfectly flat, so you can shim the mount as needed). Machined from 7075-T6 bar stock. The thickness ends up to be a minimum of 0.250" thick (which is over the FIA regulations for aluminum).
This requires removing everything in the way of mounting to the floor. For the SW20, I modified (removed and flattened) the rear seat mounting locations. I mounted this behind the front mounts (to leave the sheet metal cross bar intact).
Works for all vertical mount seats: