Post 28 show the way you install sub belts for a formula car where you lay on the belts, NOT any approved way for sedan. Go to post 40 and there is a Louts PDF attachment that show the factory attachments.
I make a bar that travels with the seat, it's a compromise between the correct belt angles and fastening to the floor. The correct angle of the sub belt is maintained though out the travel of the seat rail. The best route is to have a fixed seat location and the correct location for either a 5th OR 6th sub belt. They are different locations now, used to be the same when these brackets were designed. I still use them for both 5 and 6 point subs and feel comfortable as long as the lap belts are secured to the floor. The sub belt is designed to keep the lap belt on your pelvis so in the event of a roll over, the lap belt can NOT slide above the pelvis bone causing internal damage to organs.
2006 Elise with BOE 400 supercharger and some aero mods, CRF floating oversized rotors, MCS single adjust coil overs, CRF exhaust, CRF sway bar w/adjustable link, mono-ball pivots, machined steer arms, baffled oil pan, CRF titanium toe link brace, hardened motor mounts, Ethos wheels w/Yoko slicks