Yeah, I'm mainly a Sargo driver, never paid that much attention to the others, but I tried the Gar to see if it had the option for double five inch stubbies in 41, and it did. Anyway copied the default folder and started hacking, here's what I got;
http://www.subsim.com/radioroom/showthread.php?t=241297
The copy/paste for the Gato to Porpoise should also work for the Tench, "AcceptedTypes=USDeckGunBasic, USDeckGunAdv" is the key for double deck guns, I suspect the Narwhal also has that line.