Saw these, but I don't think this is what you want plus looks like they floor mount for the front: http://wescoperformance.stores.yahoo.net/am4point.html
IMO, there are other safety factors that come into play before the benefits of going from a 3 to 4 point belt. Stuff like no crumple zones, air bags, or side impact reinforcement etc like you have on modern cars. My point is, belts are just part of the total safety system. In a road collision there may be bigger issues like the steering column or something else, things you cannot change on these older cars if staying original.