Ditko filled in the blanks in an essay he wrote in 2008:

So Ditko plotted and pencilled the entire story by himself on spec, and provided his usual script/dialogue on a separate sheet of paper.  Stan bought the idea and rewrote Ditko's rough/suggested dialogue as usual.  The character name is the only thing Stan created (I've read another essay by Ditko where he speculates that Stan's use of "Stephen" for the first name was a joking reference to him).   

True.  But in Origins of Marvel Comics, Stan says nothing about Ditko's creation of the character.  He says he himself was inspired to create the character by an old radio serial (Chandu the Magician) and that he brought in Ditko to draw it.  And of course the original story credit itself says "Written by Stan Lee."  I would guess a lot more people read Origins of Marvel Comics than that private letter he wrote to Jerry Bails which was published in a fanzine. 

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That Ditko and Kirby had frustrations is well documented. I don't believe that is the prime reason Kirby left Marvel though. Had Marvel given Kirby a long-term contract that said, we'll look after you, we'll acknowledge how important you are, but you will carry on working with Stan in exactly the same way for the next x years, I'm sure he would have signed it. Instead they offered him a shitty contract that said he had no rights to anything and refused to alter it in any way. That's what pushed him out the door. 

I haven't read Origin of Marvel Comics, but if he said that there, that was shady.

Just 11 years after the fact (when The Origins of Marvel Comics came out in 1974), so either.....

1. Stan Lee forgot the details entirely.

2, Stan Lee and Steve Ditko were both lying when they told the original version and Stan did have a hand in creating Doctor Strange.

3. Steve Ditko was telling the truth and Stan Lee was lying when he wrote The Origins of Marvel Comics.

The two stories are just too dissimilar to be true.

Stan always pointed to his poor memory (he was 52 in 1974) which doesn't seem likely, because he seemed to remember plenty of details from several different origin stories (which seemed to evolve over time).

I have always wanted to give Stan the benefit of the doubt, but there are just some things that are difficult to accept.

From ORIGINS OF MARVEL COMICS, page 223 -

To give Stan the extreme benefit of the doubt, perhaps he really had completely forgotten the details of how Doctor Strange was created, even though it was only 11 years later.  So perhaps he started with the assumption that he created Doctor Strange and then worked backwards, thinking "what would have inspired me to create such a character?  Hmm, maybe it was Chandu the Magician?"  And he made up a story to fit.  So not so much shady intentions as a mistake driven by false assumptions.

Regardless of his intentions though, this illustrates that Stan did (deliberately or not) take too much credit at times.  In these types of discussions I often see people say "Well, it's not Stan's fault if the general public gives him too much credit.  He always acknowledged his collaborators."  Stan did generally credit his collaborators, but there are some instances like this one.  And there are also instances where he wasn't forthcoming about the amounts of work the collaborator did (ie, stories where the artist did 90% or even 100% of the plotting).  My overall point is that Stan is not blameless here for misconceptions about his role in the creative process.

The story about the creation of Doctor Strange also calls into question some of Stan's other recollections about how he created the other characters.  How accurate are his vivid descriptions of conceiving of the Hulk or Thor entirely by himself, and handing them off to Jack to draw?  I know Kirby vehemently disagreed with Stan's claims to have conceived them entirely by himself.  Are these also cases where he made up stories to fill in the blanks, regarding the creations?

Once again, there is ambiguity. He says he listened to Chandu. He says Steve drew it, first. He says he wrote the words, second. And the implication is that he put inspiration into it drawn from Chandu. And through the two of them, Dr Strange was born.

Where does it say that he created the character and then brought in Ditko to draw it?

Yeah, you are correct that he does not unambiguously say that, but it is strongly implied.

Here's his account with a bit more included which Robert left out:

He says he needed another superhero for Strange Tales.  He says he thought about his own recollections of Chandu the Magician.  He credits Ditko with "the art chores" only, and makes no mention of Ditko having any other role in the conception of the character.  He says he wrote the words (and again the story itself credits him solely as writer).  And the sequence in which he describes these events certainly implies the character was conceived first, and then Ditko did the art.  "I thought about Chandu, and then Ditko did the art chores."  In that order. 

The ambiguity is a bit puzzling.  It almost reads like he's deliberately going out of his way to avoid stating explicitly how the character was created and who came up with the initial idea.  But he certainly implies that it was him, inspired by Chandu.  When I read this book in 1976 that was the impression I came away with.  And I bet the vast majority of readers who knew nothing else of the history also came away with that impression.  I can't imagine many people reading that and coming to a different conclusion.

Well, that interpretation directly contradicts what Ditko himself said.  It's possible Ditko was lying or misremembering, but his belief system makes the former extremely unlikely.  He strikes me as someone who would never intentionally take any credit for the work of others, or lie about his own role in the creative process.

And when you see something like this, you start to question his other accounts that conflict with someone else's claims.

We read the same passage and see different things. 

I read it and see very, very little about the details of the creation of Dr Strange. No arid, step-by-step list of this happened, then this happened and then this... Instead I read the words of  a raconteur trying to add colour to his yarn in order to enthrall and amuse the reader. Bonggg! Guess what? It wasn't really the loudest gong in the universe.

You seems to get out of it that Stan is saying he invented Dr Strange. He really doesn't say that. He tells the tale massively from his perspective. i.e. he focusses on what he added. He spins a tale about listening to Changdu and how he doesn't remember any of the details, but he remembers how it made him feel. And presumably he tried to channel that into Dr Strange.

Stan's words are in this order: Steve drew it, I added the words. This seems to me to be a more honest description than you are making out. You portrayed Stan as saying he brought in Steve to draw the character he was inspired to create... and the words simply don't say that.