My eyes aren't what they used to be, so I'm also shopping for a good ghost ring or "peep" sight for my 336. Thinking about a Williams or Skinner, if the budget will allow.
Jump onto the Marlin Owners forum. There are ample discussions of peep sights on 336 leverguns. I just put one on my 336SS XLR 30-30. I had cataract surgery some months back, and can't wait to go shoot the new peep sight.