Per Crusty's pic, I guess I would fab the anchor plate behind the round hole though it doesn't look like there's too much flat area for the plate to sit flush against. So you might need to reduce the length and width of the plate, depending on what hardware you received in the kit.
I think the easiest way would be to cut a piece of cardboard that size, transfer that drawing on some flat stock, weld your nut and secure the plate on the back side of the larger opening seen in the pic.
But looking at the angle of the metal immediately surrounding the square hole, I don't see any real flat area that the piece of stock could sit against except to the left. Thus my reason to go for the hole that has some decent flat area all around. Good for welding. Some people might not want to weld in the plate so you would then consider using a couple small flat head machine screws to help hold the plate in place while installing the belt system. You could use the small hole to the left of the big hole and then drill another small hole between the big hole and square hole. Mark and drill and tap two small holes in the stock plate that align up with the two little holes in the car.
I don't see any problem with screwing it on since you are using another grade 8 bolt and tightening the plate with that up against the body anyway.
Preferred method would be airplane pop riveted to hold the plate then use the grade 8 bolt to fasten the seatbelt to it. I showed this to my friend that works for Boeing and is familiar with airplane quality pop rivets. Those rivets can hold, withstand more than we can do in a car for sure.