Those are 8"/55 Mk.15 Gun. I'm thinking to place the Mk.16 instead of Mk.15 instead.
That's not a bad idea. Since your 3"/50RF guns and electronics are obviously of post-war vintage, you may as well have post-war main guns. However, keep in mind that 4 Mark 16 turrets would add about 600 tons to your ship, and the Mark 16 guns' turrets had cylindrical turrets whereas the Mark 15 gun turrets on the Baltimore and Oregon City class cruisers were conical, so they took up less room below decks.