The round ones will go on the same backing plates as the square ones, there is no difference in the bolt pattern at all. Also, the spindle section where the hub rides on bearings is exactly the same length and dimensions on both. You can interchange them back and forth with zero fit problems, but the kingpins for the squarebacks are slightly longer than the roundback ones. Don't know why Ford changed, but they did. For some reason the roundbacks command a better price than the square ones, not sure of why that is a factor, but it is generally the case.

Don