If all is smooth, no mods are required. If the lower tree / stem still wobbles around and you have fore and aft play in the assembly, then you will need to move the shoulder back.
This does NOT have to be precision. It is clearance only. Mask up the bearing race to protect it and use a file or Dremel to remove material from the problem area. If you get it a little smaller here, it doesn't matter. It's just so the bearing can seat in it's race and not against the shoulder on the stem.
