Our batteries are connected like this:
(Except with 8 instead of 6).
You might want to check the information about the maximum battery back that can be created. I KNOW that they do not recommend connecting more than 4 in SERIES (as you would if you were making a 48Volt bank), but I can think of no reason that the number of parallel batteries would be limited. My manual doesn't suggest any limit.
I found the terminals of the Firefly G31 batteries to be slightly taller than the Lifeline batteries they replaced, but our fitting isn't tight, so a bit of expansion in footprint I would not have noticed. Generally, for any battery, it is not a great idea to have them pressed tightly together. They need a bit of air space to lose heat.. Firefly suggests a minimum of 1cm between batteries.
I might be labeled a heretic but do you really NEED 12 batteries? We have never been in a situation where we felt we wanted more than the 8 our boat came with. The Firefly batteries are happy with a deeper discharge profile than the FLA batteries the 54 was designed with.
Annapolis, MD, USA