From what I've read of the history, a lot of the first generation 4runners came with cheap bolt in seats only to lower the cost of importing them; the two-door, two-seat version was classed as a truck and had a 25% import tax, while a two-door, four-seat version was classed as a vehicle with something like a 2.5% import tax.
Some of the 4runners came with what's been called a "seat delete" panel, which is basically a thin piece of steel that would go where the backseats are and makes it look more like a truck bed. Most of the dealerships would take these out and scrap them to install the back seats. I've read a couple of threads about it between yotatech and pirate4x4, so if you go to either of those sites and search for "seat delete panel" you can probably find more information.
I've been messing with the back seats in my '87 a little bit lately. I've got the headrests, the problem is they don't come out at all, which means they get in the way when I pull the bottom out and try to lay the back down. I got some backseats to a '90 4runner, and they almost bolt right in. The problem with them is that the locks on the back are in a different spot than on the first generation. The advantage is that the headrests actually come out so I could lay them flat if I wanted to. I think in the end I'm going to have to take my original seats apart and find some way to take the headrests out without destroying them.