Hmmm, what's safer, being 12" nearer one door in a Ford-designed & installed propery 3-point belt, or being 12" farther away in a jury-rigged 3-point belt?

Unless the car is hit square in the door where the child is sitting the factory belt is the only way to go. Also, no guarantee that even in that crash the jury-rigged belt wouldn't fail and cause more injury. If you really want a center shoulder harness that badly, sell your car and buy a newer one.
Just my $0.02.
Scott