Whoa there, podner.:B
Before you make any big changes, you should know that your new 6V's are not going to be in a fully active state so the SG will be less for the first few cycles. You also started at a very significant discharge level. With 400+ amp hours available you may never see that DOD again.
Yes, 6 AWG is too small for the lengths involved. If you have enough #2 to wire from the converter to the battery, that would be a good choice. I think, with a wire upgrade, you could make this all work, no need for auxiliary chargers and such. Of course, a lot is dependent on your use.