Well, I installed one using the existing points, given that my welding skills are lacking. The Buckle behind the shoulder seems smooth enough, but given the inset at the shoulder point location, it's not at the right angle to work optimally. I'm thinking about getting a bigger bolt and a short sleeve to extend the shoulder point out a bit so it can swing free.
I can definitely see the advantage of changing the mounting point for the retractor, but doesn't look to easy.