Not sure I agree with the notion that if the factory belts use the bulkhead than it should be good enough for aftermarket shoulder belts. The factory 3 point mounts at the outboard extreme of the bulkhead and the bulkhead is strengthened and braced in triangulation for a forward load. Inboard shoulder mounts will not benefit from this structural reinforcement. In fact, towards the center of the bulkhead you can see it is more flimsy. Use of spot welds in the center section. As I recall from removing this bitch, the outboard reinforced area had some stitch welds. Much stronger.
Also, roll hoops are designed for a different load path. Pushing down, not bracing a forward load.
Hopefully this picture helps illustrate: