I think the '76 is the W115 body style, so this may be different for you.
In my '83 240D (W123 body) you put the rear seat cushion back into place in three easy steps:
1) Move the seat belt parts out of the way. I usually wedge them in next to the armrest while the armrest is folded up.
2) Slide the seat cushion into place, pushing it toward the rear of the car as far back as it will go. Make sure to line up the little orange plastic tabs (bottom front of the seat) with their corresponding holes.
3) Climb in the car, sit on the seat, and bounce up and down a few times on each side. (depending on your body weight, I'm a skinny little sap so it takes a few tries). You should hear a reasonably loud "CLICK" on each side as the holding tab snaps into place.
'83 240D, 250k miles