I'm not familiar with the Dolphin kits so I regret I cannot answer which of them is best for the conversion.
No plans to design the deck gun. Another vendor, diStefan, offers one in other scales. Please contact him through his Shapeways shop to see if is he is willing to re-scale his design to 1/400 for you. Link:
No plans for open rangefinders for Bismarck or Prinz Eugen. Bismarck open rangefinders are offered by diStefan.
Hope this helps and thanks, Peter!