Looking at your photo's, you might want to consider routing your belt like this...
Attachment 176300
You have a double wammy in that you have very tall seats that are not designed for a 3-point belt system.
If the goal is 5-point belts, maybe replace the seats you have with a fiberglass shell seat and mount it right on the floor (no slider), but you probably will still fail a broomstick test with the FFR OEM rollbar.
A better option might be replacing the seats you have now with seats that accommodate 3-point belts and fabricating mounting brackets without sliders that get them as low as possible.
My passenger side is an OMP Designer tube frame seat. Super comfortable and 3-point compatible. Fine for the occasional track day. The Corbeau A4 looks very similar. The driver side is a Sparco fiberglass shell seat. Crazy low. Not designed for a 3-point belt, but I have a broomstick rollbar system (front and rear roll bars).
Attachment 176301