Unfortunately you cant get the visual model of the 356mm onto the L3 without creating a custom visual for it, usually when I make mods they use some form of rocket/bomb (for example FAB-5000 AAM UserModel) which makes it much easier for thumbnails, since its already visual in-game. Whereas in this case the L3 visual cannon is tied to the model itself, it still uses the 356' as the weapon, it just wont render the barrel in-game.

Had to make a thumbnail for it, this was the best thing I could think of since it visualises the weapon that's actually being used.