Dear Mr. Clay: Although the exact facts of the matter are as hard to nail down as in any other Hollywood tale, I believe the story is that Welles was working on THE MAGNIFICENT AMBERSONS "in his day job," and doing JOURNEY INTO FEAR at night. Obviously, sometimes Welles could not be there. Also, Welles had obligated himself to fly to Under Secretary of State Nelson Rockefeller to fly to Brazil on that infamous "Good Will" mission after shooting on AMBERSONS was complete, but before JOURNEY INTO FEAR was done. By default, the job and the credit fell to Norman Foster. I also think Joseph Cotton, who co-wrote the screenplay, took a hand.
[Somewhere, I have a rather sad article by Foster's son, saying that Foster was willing to drop anything he was doing to go help Welles -- and did!]
Hope this helps.