Thanks to all who have replied to this thread. Yes, I am planning to put in a new rear seat. I purchased one with headrests, because I want to protect my kids against whiplash. The seat I purchased came with a bracket that mounts over the bolt holes for the existing fold & tumble bench seat. The front bolts, two on each side, came out fine. The bolt holding down the rear bracket mount, along with the lap belt mount, was stripped when I tried to remove it.
I have been told that this bolt is directly over the gas tank. I did not think about the rear differential. I like the idea of welding a bolt head to the part of the bolt that is sticking out, but that seems rather permanent.
Ideally I would like to have it drilled out and insert a new bolt. I welcome further thoughts on this.
And if anyone has a sense of what a decent OEM rear fold & tumble seat is worth I'd love to know. I am going to sell the one I just took out of the 1991 YJ.