You are half right. The upper A arm is unique to the 57. It wraps over the frame rail. The 58 and 59 had the mount on top of the frame rail. The lower A arm, however will work on the 57. It does, however, require the spindle from the 58/59. So, you can do the lower arm and ball joint swap and save a bunch of money. I actually bought a 58 suspension intending to do just that, but the lower arms were so rusted I chose to just keep the 57 ball joints. BTW, that is one very difficult and time consuming job. All the bolts are close fitting in the the holes holding everything together and I had to heat with a torch to get them out.