Making the mounts for seat belt bolts should work if you are relocating seat belts and they are easy to make. As long as they are tightly welded to the floor it should be safe.

Nice idea !  
Would the seat mounts be set up like what you would do if you are changing the seatbelt mounts. Larger plate on the outside with a higher tenstile bolt. 
If you have any blogs on such a project that would be helpful. 
 
Ausgoat