The Ford 9" have two different ends. One is what they call a big end rear, the other a small. This you need to know from the start to get the correct brake parts, either drum or disc.
One other thing when buying a used rear..........make sure it is straight!!. it can be out a small amount and you will never see it. But if you take it to someone who knows what they are doing, they can tell you if it's straight or not.