There is a threaded hole on the roofline where the original shoulder belt attached to. I installed a retractable 3-point belt in my Challenger with a similar layout. The The retractor sits on the floor and the shoulder bolt is the other attachment.

I would think that the dart would be similar to the Challenger. I guess I mostly would like to know what manufacturer to go with since I am not happy with the belts I got for the Challenger (I can't remember who made the Chally ones).