LD has used different gauge wire, between the converter and battery, through the years so, when upgrading the converter, confirming the actual size used is necessary.
To determine the wire size needed, measure the distance of the wire run, from the converter to the battery. Use a wiring chart showing the amperage capacity for various gauge wires and run lengths. West Marine Wiring Size
In the above chart, use the 3% voltage drop and make sure to double the run length to allow for the positive and negative wires. The correct size wire, or larger, will prevent excessive voltage drop and provide the highest possible charge rates.
There is no rule of thumb that a 45-amp converter should use 6-gauge wire. Using West Marine's chart, a 45-amp charger, with a run of 12' or more, should be upgraded to 4-gauge. Every floor plan has a different wire run length.
The wire size is a function of amperage and wire length. If the wires are bundled with other wires, the wire amperage capacity is degraded, and may need upgrading to prevent overheating.