See this post and this post (and links from them) for good options in 3-point belts. Do not install a 4-point belt/harness setup if you value your life and intact body; see here (and links from there).
When I read your posts, Dan, I get the impression that your study first, the apply some research, and after THAT do stuff. I like that apporach.
Still I am not sure that a three point belt will do good (or greater) in this particular situation. If anchor it to the body behind the C pillar, the angle will be low and the seat will be in the way. The hole car is not design for belts at all - or a collision...
And the more I read, the less I want to have the kids nowhere near the Dart... :-/
