I thought of replacing the rear seat of a 500E with the rear seat of another W124 sedan, but I wonder if the 500E is US DOT rated to be a 4 seater or 5 seater. It sure looks like there are only 4 belt sets. Adding a center lap belt adds a few more fasteners to the job of replacing the seat, but it could be a sticky insurance situation. I don't know if it's a simple matter to recertify the car.
I should give up the hot rodding dreams and 'settle' for an S500