I used the stock locations on the lap belts.
On the sub belt, I welded a 1/2" nut to a piece of 2.5" x 6"x 1/8" thick flat stock. Using a 1" holesaw, I cut a hole in the cross car brace that runs under the seat. I welded the flat stock to the brace with the nut inside the hole. Nice and strong.....not likely to fail