I think others answered this for me already, but there is no way you are getting an ship much over midway size without making the flight deck the strength deck. adding expansion joints in the deck will thus not be what you have to do for your 1200 foot carrier: you will have to restructure everything below flight deck level. as said: "Starting with the Forrestal class, the size of the carriers meant that stress requirements forced the abandonment of the external hangar and hangar deck as strength deck concepts". this has nothing to do with the year, you simply have no choice. using the midway class style construction, the ship would most likely flex over it's limits the moment it leaves the harbour, and even with hindsight we have, we can say that designers would be smart enough to prevent that. that much knowledge would be there, otherwise the liners of the era would have similar issues.
note that the gallery deck under the flight deck was IIRC also the space where the briefing rooms and ready rooms were positioned.
I have done a quick read, the reason for the british having the flight decks as strength deck was the fact that that resulted in an armoured flight deck in an as light as possible hull. note that while the british carriers were a failure in wartime operations, the USN started building armoured flight deck carriers postwar. the midway was the first, but there it was late in the design this was added.
all this said, after you have fixed the design, I really hope for an angled deck conversion with modern fire control and aircraft. 1950 or something like that seems likely