Al, the 6V GC batteries are interesting. If the box is tight you could only get two into the box and you loose a little capacity and waste some space. If the box has about 5/8" extra length you could get three into it and gain a lot of capacity. You also need a little more height. If you've got a little extra room, this sounds like a great option. I think the wiring would be a little complicated. How would you do it? Two in parallel and the third in series with the first two? If like me, you've got 4D batteries, two GC batteries are also a great option. Length and width are fine, you need an extra 7/8" of height. My batteries are mounted on their side so width becomes height and height becomes width. I've got the room. Next time I replace my batteries I think I'll try it. The dimensions I've been using came from East Penn Deka batteries, Intimidator AGMs. Here is a link to the chart:
http://www.dekabatteries.com/assets/base/1740Intimidatordeepcycle.pdf