I added belts to my '41 a coupe years ago. The seat belts are bolted through the floor and I added a piece of 1/8" steel under the floor for reinforcement. I welded a piece of 1 1/2" square tubing to the beltline in the back of the cab and welded the shoulder belt attachment plate to it. The retractors are mounted on the floor. I used aircraft style buckles & I think they came from Seat Belt Pros.
