I have 2 290-series deep cycles in my bilge ( deep draft ) on shelfs built by the PO. I was going to move my shower sump and make room for a 3rd just behind the mast, but my wife suggested I put it in the starboard storage under the couch. I think thats an excellent idea, and would actually hold 2 batteries. I also think that side of the boat is light, compared to the port side which has a full-length water tank and the diesel tank. Should even it up.
Once its done I will have 4 290's for the house bank, which should give me about 420-440 amp hours. How many amp-hrs will you get out of those LT16 6 volt batteries?
I'm also curious if other 37c owners use that space for batteries. There is already a hole from it going into the bilge for the water pipes, so it seems a good idea.