The Seat belts need to be mounted to or underneath the seat. However, make certain that they go through a frame member. There should be one close in the car you have as it came with it’s own steel frame. If you need to weld a support bracket – then do so. Under no circumstances rely on just bolting through the fiberglass floor. Even with large fender washers.In the case of VW owners the pan is not sufficient by itself.
After all is said and done the real area of concern is the windshield. Generally you have two 5/16″ bolts going through fiberglass holding up a big wind catcher. Post Caps Help by adding a couple of extra bolts and the Fiberfab MIGI had a steel support plate in the mounting area. CMC did not.
Part of CMC problems were brought on by a decapitation law suit they spent hundreds of thousands fighting.