73 Swinger Ford 9", GM 10 bolt?

The rear end you are looking for depends on what you plan to do with the vehicle (street car, race car, or a combination of both). If you can't find an 8 3/4, the 57, 58 9" is almost a drop in. Stay miles away from the GM 10 bolt, uses c clips for axle retainers. You might also look into a Ford 8.8 with disc brakes. But as I said, it all depends on what you are going to do with the car.