Hi Alec, I would go to the next larger size. Think of cable like pipes and flow of electrons. Even though it's the neg leg, most of the draw will be thru the positive side, upgrading the neg side will not hurt.
I used 2/0 for my car amps power distribution blocks.
This is also a great time to fit an isolator switch, if one is not already fitted. Whenever I put my Rolls in the garage I flick the power off - great insurance against fires on a car with 45 year old wiring.