I plugged in 6 feet round trip, more than you have, with a 2% permissible voltage drop and 200 amp load and it comes back as 2ga.
Online Wire Gauge Calculator (LINK)
I really don't think you'll need to do anything. I have owned 6 boats with 6V banks and each boat was wired series/parallel with either 2 Ga or 1 Ga and I never had an issue. For the length it was clearly over sized..