I thought something like that could be done here in Boston: a land bridge to connect East Boston with the main part of the city. Though since that channel is the outlet for the Charles river, it would need to be raised so that water and shipping could get through.
That's a cool idea, I wonder if there were any secret proposals for it in the 40's. I was shocked that I could grow up in the SF Bay and not know about this until I was in my late 20's...
Golden Gate Park has plenty of room.
They could at least re-zone it as a state camp-ground or something...
Just like in Man in the High Castle :D (or Atlantropa...)