I installed 3 point retractable belts in my 54 panel. Instead of getting inside the panels I surface mounted 3/16 steel plates in the proper places. Each plate was about 3X5 inches with nice rounded corners. The seat belt mounting bolt size was 9/16 so I drilled into the plates and welded a 9/16 nut on the back side of the plates. Then I cut a hole larger than the nut into the truck and surface mounted by welding a continuous beed around them. I cleaned up the welds and used a little glazing putty to dress them up. Not original but either are the belts. It's a very strong connection provided it is welded correctly. It also looks good and will hardly be noticed with the belts hanging in front of them. Good luck.

All the best, Ted