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