After further review and discussion on a few different forums, I will be purchasing two 6V AGM batteries from Sam's Club for $180 per battery as advertised HERE
. I understand AGM technology is maintenance-free, charge more quickly, and has a longer life cycle compared to standard deep cycle lead acid 6V and 12V batteries. Time and experience will tell if my information is good.
I will remove my roll-out battery tray and screw down sheet steel for the bottom of the battery box for adequate clearance for the taller batteries. I will come up with a means to cradle the batteries so they won't move around inside the big box. It will be nice not doing any maintenance on them. Just mount them and forget them until they no longer perform.