My BCW Chevette based kit uses the Chevette belts. They are mounted though the floor and anchor directly to the frame. The outboard belts mount to a 1/2″ nut welded to the frame rail. The inboard belt mounts to a steel bracket also welded to the frame.
When you mount your belts, whatever you use, be sure to mount them to the frame NOT THE SHEETMETAL FLOOR. If you do mount them to the sheetmetal use huge fender washers (2″ O.D. or larger) on both sides of the mounting bolt.
Also, do not cut and re-sew the belts to shorten them. That is what killed Dale Earnhardt. It is better to use one of those adjustable, buckle style anchors.
The grim reality of these MGTD kit cars is, that even with the best belts, if you get hit or hit something, you are in big trouble. They are really only 4 wheeled motorcycles with an MGTD shaped body faring.
In the future, I plane to mount a roll bar in the Pink MG and add shoulder belts to a cross-bar between the roll bar upright tubes. I’ll replace the current Chevette lap belts with a 4 point system.