Convertibles with 3 point seat belt conversion?

I have thought of doing the same thing on my 69 Dart vert. I think there are a few threads on here regarding this topic.
Here is a link I saved from a company who makes "convertible aftermarket seat belts"

Retractable Aftermarket Seat Belt - 3 Point Replacement Seatbelts for Convertible Car Hardtop Older Camaro, Mustang, Corvette, Falcon, Firebird, Nova Seat Belts