FYI. I have the Sabelt clip in harness (Made by Sparco).
It works great! I had to make a mounting bracket for the lap belts to clip to, and the it uses the existing rear seat belt anchor for the shoulder straps.
Here are some pics of my set up so you all have an idea of it.
Lap belt bracket Rear anchor How it looks I hope this helps your GB out. If anyone needs info on making the mounting bracket, PM me.