I installed my own Sparco seat this weekend and wanted to add my .02. To top it off, the resistor method described works perfectly.
The entire wiring harness from under the seat has to come out. It's not tricky, just some Christmas tree clips and such and it will come out. My Sparco seat had quite a bit of webbing on the bottom, so I was able to tuck the magnetic sensor and other wires into the webbing to keep them safe and secure. All of the plugs go right back where they were once you have the harness off of the seat, EXCEPT the sensor that plugs into the air bag in the actual seat. That is the plug that you have to splice in the resistor.
The planted bracket has a place for the female end of the seat belt. You will need a M10x1.5 nut and washer to secure the female seat belt end to the bracket. If you are using a reclining seat, that's it. If you are using a fixed back seat like me, the housing around the belt keeps it from going into the hole in the seat. I used a razor and cut off most of the plastic housing so that it would be "floppy". This exposes the wires, so I wrapped the whole thing in electrical tape to protect it.
Even with rails, this drops the seating position quite a bit.