I did the same project on my 56 a couple of years ago and had some of your same issues (I still have one shackle on the rear that I have been unable to remove). My original shackles had just the single bolt holding them together, so the parts from LMC matched at least. The bolts on the bottom of the step are called carriage bolts and you should be able to find them at a good hardware store.
Lee.
