Ed,
I just finished installing four new 6 Volt batteries under the quarterberth. They are high-capacity gel cels weighing in at 72 pounds each (!) and are tall enough so that they don't fit the original compartment. Actually, only one of the batteries was too high (the aft-most battery), but I solved several problems at once by raising the quarterberth bottom above the batteries so that the q-berth is now flush with the teak around the front of the area. I have long wanted to sit higher at the nav station, plus I wanted more and better organized storage under the quarter berth. The new seat height at the nav station height feels much more comfortable and the quarterberth bottom now has three large access panels to the storage under -- the improvement in access is wonderful. It's also very nice to now have a 370 AH battery bank for the house. A final bonus came in the form of the battery hold-down design. I built a shallow tray that exactly fits over the top of the batteries and, besides restraining the batteries in case of being rolled (yuk!), it serves as a convenient place to keep my general boat maintenance tools. The tray is pinned in place with six fast-pins so that I can remove it and get to the batteries quickly if ever necessary. The four Dynasty gel-cels have large rectangular lugs to fasten the 2/0 battery cables to, and since the batteries never need water, I doubt I'll need to pull the tray and inspect the battery terminals for corrosion more often than about once a year.
I have some photos if you are interested in doing something similar.
As long as I'm on the topic, next I plan on adding a single 12V engine-start battery in the engine room and then hard wiring the house bank as a single large bank. This is the most efficient system for a cruising boat since the charge acceptance rate of a large battery bank is great enough to use the full output of a large alternator, and recharging the batteries less often will make them live longer (unless you discharge them too deeply before recharging). The bottom line is you can't have too large a battery bank on a cruising boat.