That era of car, I don't know if there was a consideration of beefing up areas for seat belt mounting. If it was me, I'd just pick a spot that puts the mount area out of the way of back seat passengers' feet. Be sure to put a lot of sealer on the bottom side of the mounting holes. While the sheet metal was heavier then than now I'd still reinforce the holes with large washers.

Thanks, yes would even consider a beefier plate underneath for reinforcement. Good suggestion about room for backseat passengers , didn’t even think of that!

I would also round the edges of the washers. That way if the unwanted does happen the square edge of the washer doesnt start cutting through the parent metal.

Tony

Take a look here for a few installations. This should help.

https://www.julianos.com/Articles.asp?ID=143

https://www.julianos.com/Retractable-Lap-Belt-Anchor-Kit-for-Bench-Seat-p/ju010800.htm

later, bill 

You may also look for the floor bolts that actually extend down through the floor and through the frame. If those just happen to be just behind the front seat,  you could make a frame that connects a crossmember from one side to the other. 1" x 2" square tubing works fairly well and then all your belting can be mounted to that. If you want a shoulder belt too (which I would recommend) you could install a roll bar mostly for the seat belts and less for the rolling. It'll take some thought but it's very doable. When seatbelts first started getting installed in earlier vehicles, lap belts were all they did. Later when shoulder belts became commonplace everyone realized just how worthless lap belts alone are. You don't get thrown out of the car with lap belts... and that's good. But you still get slammed into the dash and windshield... and that's bad.