You are right. Seat belts in a 71-73 Pontiac Grand Ville, Oldsmobile 98, and Cadillac Sedan DeVille/Calais, will be the same. (If it is a 4-door hardtop), 2 door may be different. I don't think Chevrolet will work. I have a 71 Caprice, and even the top of the line Chevrolet has the roofline like the LeSabre, Delta 88 and Catalina/Bonneville (with rounded rear window) But, seat belts still may be the same, because the belts are mounted in the same place on these cars, and the buckles are identical. Hope this helps.