Davey,
Here are some pictures that might clear the location of external tubeson on Salmon and Sargo.
I am going to guess here and say the best place to put the external tubes is as close to the torpedo loading hatch as possible. I am further going to guess and say that they would want to do it for the forward room.
Skematic of Salmon/Sargo:
Note the position of the forward torpedo load hatch as opposed to the gun support.
Picture of the Seal's deck forward of the Conning Tower:
Notice the two Hatches to the port and starboard of the gun. Also take a look at the rail and deck expanding the width of the deck just forward of the gun to accomadate the hatches and not the gun.
A picture of the deck aft of the Seal conning tower:
Notice that there are no cuts in the deck for external torpedo load and Look at the four rises along the edges of the deck to accomadate enlarged engine mufflers. The aft torpedo loading hatch is just past the last set of deck rises. No visible sign of cutaways in the deck to store external torpedoes and no room to put em because of the mufflers.
Last pic of Squalis construction:
Notice the big cutouts in the deck with the wooden planks over them. The picture is looking aft from the bow.
A picture of Seal after a 4 inch deck gun was added:
Notice after adding the 4 inch gun the cutaways are no longer there and the deck has been widened around the gun where it wasnt in early pictures of the Seal.
It would appear from these pictures the external storage tubes were forward of the Conning tower and they were removed when adding a four inch gun. Well at least for the Seal.
Anyways thats me story and I am sticking to it.