Rear shoulder belts?

The factory shoulder belt option was for the front seats, not the back. Anything you do will be suspect from an engineering point of view, but with that said we are planning something similar for my son's Dart convertible. One of the things we need to go get is one of the bars that the car seat attaches to at the bottom (his kids a 3 and not yet 1). After that we'll look at some of the factory upper mounts and either duplicate them or swap them in.

As for three point for older kids in boosters, we haven't thought about that yet...