Pony Car Concepts sells a 3-point belt kit that should work in convertibles. The shoulder belt mounting point is attached below the rear passenger windows, similar to some modern convertibles. They don't have a website but you can phone for a brochure at (714) 639-4457.
I home-engineered something very similar using a generic 3-point street rod seat belt kit from Juliano's street rods.
I remember seeing (or imagining?) a few years back about a 4 point belt that used the existing front and rear belts. The over the shoulder part plugged into the rear belts and draped over the back of the front seats and your shoulders and then connected with buckles into the existing lap belt. This would be very cool since it would allow you to remove it for shows and you wouldn't have to cut and weld on your precious interior. I haven't seen this anywhere recently, like I said maybe I imagined it!
If anyone has seen this, PLEASE let me know! Thanks.
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