I don't believe that the 707-436 ever flew scheduled non-stop LHR-LAX,
You can believe all you want, however...you, yourself, would be quite mistaken.
Sorry, them's the facts.
Payload restricted, it most certainly is possible.
And...done, on numerous occasions.
Confirmed, by those in the know.
The rest?
Guessing, no more nor less.
No surprise, there.
