How did you attach your 3 point retractable seatbelts?

Did you ever get your seatbelts to work satisfactorily? Did you end up having to do modifications to be able to use them? If so what were they? I have a 71 Dart ( bucket seats ) that I want to put a 3 point harness in and I am now in the process of deciding which kit and whom to buy from. The prices for the kits are all over the map ( $200 to almost $500 ). I would also like to hear from anybody for advice on which kits seem to fit and which need modifications to install, I really would like to buy something that bolts right in without reengineering. I feel shoulder harnesses are the single best safety upgrade you can do to these old cars to make them reasonably safe. Anybody that is running only lap belts needs to watch the You Tube video of the 1200HP Mercury hitting a car in the rear at a stop light to see how well a lap belt works by itself.