I have a couple of photos in photo bucket of how I am doing my seat belts (lap only)in a 46 Art Deco. The cab has a second set of holes at the rear mounts. I used 7/16in grade 8 bolts with a fender washer underneith. I spaced the angle above the cab mounting springs with pipe. I used a 3/16 in 2by2 angle iron. I bolted the seat belts with grade 8 bolts to the angle.
From your description and the photos, it sounds like the fender washer and nut are the only things supporting the belts underneath the floor in the event of a collision. If this is true, you should use either a length of flat plate of angle iron (best), to spread the load over a greater area. If you're in a collision, the fender washers will tear right through the floor and your belts will do nothing.
