Cool, glad to meet your approval. I wouldn't go with C-shaped ribs as I think they might get too much in the way, these angles clear the floor a bit. (of course, I don't know how deep a C you mean). Besides, this is sort a tip of my hat to Matt Jefferies, who used this sort of angle alot in his designs. In fact they way this shape is slightly rounded where the upright meets the ceiling beam is taken directly from the Botany Bay in "Space Seed" which further attaches this design to the overall continuity, but in a subtle way most people probably won't pick up on.
I didn't want to put any sunken areas because if Kriq' wants to actually build this, that means building a whole floor on top his perfectly acceptable stage floor. I'm trying to balance what would look cool with production realities.
As far as the width of the bridge, by the original scale I designed the ship at, based on Kriq's specifications, the part on the ship model were assuming is the bridge is really something like 30' wide, making these sets take up about half that space, centered. I was originally thinking about putting side rooms on this set (a ready room and a briefing room) but figured that's more than there might be room for on his stage floor. So yeah don't worry, there's plenty of room for an elevator and a hallway. (I haven't seen "Down Periscope" in forever though so I forget if there was anything special about the sonar station.)