I'm no expert on wet cell batteries, but when your GC batteries are down to anywhere near 50% it seems like they should accept way, way, more than 5 amps with the alternator maintaining 14.1 to 14.2 volts right on their terminals.
Since you are (probably) using two 6 volt GC batteries in series, all six of their cells are in series and it would only take one cell to be going bad with too high of an internal resistance to throttle total acceptance current down to the 5 amps that you are seeing. If this is the case, more/larger cabling from the alternator would not do anything.
Just a thought.
