Here is a more detailed view of what I'm talking about
: The Argonaut had the protusions in the hull for the deck gun platforms fore and aft of the conning tower, but
at the same level as the deck:
The Narwhal/Nautilus had the same protuberances (may be a bit smoother) but also
a raised platform that placed the deck gun not at the deck level, but raised over it.
You should therefore use the Argonaut as base, with the wide deck gun platform at the heigth of the deck, and then put a platform over it to get the Narwhal. You could have the models for the same effort by simply making the Argonaut from the earlier stage of the Narwhal.