So far I have only installed 2-6volt golf cart batteries.
I have read, and viewed a picture of one persons' set-up that shows 4-6volt golf cart batteries and one 12volt between them. The 12volt is long and narrow (similar to what used to go into some 1940's Pontiac.
I will be doing some measuering in the near future.
As to putting that much weight in the battery box, I don't see a problem. I've been under and looked at the structure of the box and in my case I would go ahead with it. (I think there is enough room for 2 Group H's) Not sure that you could get them in, but looks like it is big enough. LOL