With a 6' (12')run... I would have a fuse on the pos line (maybe two, next to the batteries on each end).
How much amperage do you expect to pull?
I assume the battery charge and draw points will be on the old batteries?
More Amps will be drawn from the closest batteries esp. if the cables are smaller (more resistance).
You will need to have really thick wires 1/0, 2/0 or even 4/0 as mentioned above.
You CAN double up the wires per pole, the math gets a bit wonky, but google 'dc voltage loss' and use the calculators and play with the wire gauges and expected amperages to get to a voltage loss your comfortable with.
(remember to fuse the wires at the max you want to pull, not to exceed the wire maximim)
