I have three point belts in my '37. I wouldn't drive one without three point belts due to the steering columns. You are not going to find a bolt in kit, you will need adaptors, spacers, risers, and what ever else you need to make them safe.
When my truck was in primer, no interior, I made a rear cross bar out of heavy wall rectangle tubing that follows the sheet metal around the back of the seat behind the interior sheet metal, it's just above the rear window. This cross bar is welded to the door frames and rear window frame. The upper seat belt mount is bolted through the cross bar, it's made so all the force from the seat belt will pull on the whole cab back and door frames, not just one spot on the door frame. The lower mount with the retractor is bolted through the seat riser and floor board with heavy duty 4 1/2" washers under the cab. This again makes the belts pull on the whole rear section of the floor and seat riser. For the buckles between the seats, they mount on 1/4" wall tubing, just behind the seat riser, with heavy washers under the cab. All mounting bolts are 7/16 fine thread with locking nuts. Working in the school bus industry mounting wheel chair lifts and floor mounts, I learned about safety and how to properly mount parts. You must spread the load across as much structure as possible. Do not use the frame rails, if it goes real bad, you don't want to be tied in by the frame with the cab loose. I also was drag racing for years and followed NHRA safety protocol, I seen enough crashes to know safety should be number one priority.
I will post some pictures when I get to my main computer later today.
