So, here is what I have been told by GW - and I'm no battery expert. The market mainly has starting, deep cycle or dual-purpose batteries (plus some other sophisticated variants). Starting will have higher CCA and deep cycle will have longer reserve. For my year boat, they recommend the dual-purpose which has some features of both. They specifically said I would have charging and longevity issues if I used the starting-only or deep cycle-only or if I purchased the newer gel or AGM batteries. With dual-purpose regular maintenance free batteries, the motor charging system will not care whether you have group 24s or 27s.
On my 265, still working on it but I think I will have to trim the aft vertical support piece about 1" to fit in the two group 27s on the starboard side. Definitely will be getting new trays - the no-metal kind with the strap. I would NOT trim out the old 24 trays to make the 27s fit because if you do, the small amount of seepage you seem to get over time will leak out of the tray and all over things down there. My battery charger is mounted starboard side, aft, up under the wall just above the dual batteries. The plug is run to underneath the starboard gunnel, aft, facing forward with a cover.
I also took some notes and wrote down how the battery switches work and include here for your reading pleasure:
Guest Switches port/starboard tie to motors port or starboard.
On switches, position “1” means that switch(motor) is started from and is charging the starboard battery bank when running.
On switches, position “2” means that switch(motor) is started from and is charging the port battery bank when running.
On switches, position “both” means that switch(motor) is started from and charging both battery banks.
Preferred position is one switch on “1” and the other on “2”. You can/should select and switch between the two periodically through the “both” position while running - just to even things up. Never select and switch positions while running through the “off” position or serious engine damage will result!!