Here is how mine are mounted in a '37 Chevy cab. The upper mounts are heavy wall tubing cut to fit and welded to the cab frame and rear window frame. It was the best I could come up with, it would have to pull on the whole back of the cab before it will come loose. The lower outer mounts go through tubes that are welded to the seat riser and floor pan. The two center mounts are bolted to heavy wall tubing that is bolted to the floor pan with 4" washers and reinforcing plates on the under side. I used a standard 3-point belt with retractors from one of the suppliers who sell belts, nothing to fancy, just good solid USA made parts.
