I've crawled in and out of a number of wagons in junk yards, pulling out third seats. Seems there were a number of different ways they were secured, in different years. The one I put in my 1984 I yanked off a 1985 or 1986. It's been a couple of years, so my memory is fading a bit, but I didn't see any kind of bolt or hole in the center, down to the frame of the car. I pulled the whole plywood piece out of the donor car, with third seat still attached to it, unscrewed the hinge of the plywood piece in my 1984, and screwed the new plywood piece in place.

Instead of drilling new holes for the bottom mounts of the seatbelts, I doubled them up on the existing anchor bolts for the rear seatbelts. It's not factory, but it works.

I wouldn't have put my kids in this seat if I didn't think it was safe. Never occured to me that in a bad accident the whole plywood piece could rip loose, with kids still strapped into the seat. Is some additional security needed (besides the steel cables which bolt to the frame near the tailgate)?