I noticed an ad (P.E.Products, www.peparts.com, 800-593-8787) in the latest Roundel for seatbelts for older cars. I called them up and they will repair my broken seatbelt (73 3.0CS). Basically, for $150 (including S&H) I send them my broken belt assembly and they will sew the male buckle to a new belt/inertia reel assembly. This is great news, now I can keep the original belt configuration and do not have to do mods to update to the 74 style belts.
So, if you have a seatbelt problem, give them a look.