Answer:
This one is open for for opinion. I've seen this movie a few times and Eastwood was "The Stranger". Nobody ever called him by name in this movie. At the end the guy etching the tombstone says, "I never did know your name." At which The Stranger replies, "Yes, you do." The name on the tombstone is Marshal Jim Duncan and that gives a belief that he is The Stranger who has come back to help this town.
It's also said in his interview on "Inside the Actors Studio" that the original script was the brother of the Marshall Jim Duncan who was killed. Even in the movie he taunts Stacey by saying the same words of his brother... "Help Me".