I had to change out the battery trays and I fit four group 31s in the same compartment. Admittedly they are tight but they do fit. As for the battery set up, the way Grady-White sets it up seems to me to be a lot simpler and adequately redundant. I have the alternate charging cables that are available on higher horsepower Yamaha‘s that crossover in case you are running single engine you will still be able to charge two banks. I like to think of the battery switches as attached to the engines and not the batteries. I agree that the house battery is best as two parallel batteries. However with Grady-White set up you can have either of your battery banks supplying the house or as back up for the port engine. I believe the way Grady-White sets it up is the simplest and just as redundant as a more complex set up with two separate starting batteries and a double bank for the house, Nice to have either parallel bank to power the house, run the windlass, bow thruster, start the grenset, or start either engine. The stock set up has no requirement for an ACR. My GEN set runs a pro mariner four bank charger with leads to each battery that was recommended by the engineers. I have considered Other combinations such as you suggest, but see additional complexity and no benefit with regard to redundancy. My set up allows me to charge batteries even in the event of a single engine and always allows for a dual bank to supply the house. I guess having three independent sources of power versus only two is of some benefit but the complexity is a disadvantage I believe.