The actual thickness of the wire core on my 1984 300D was 1/4 of an inch. There is a picture in my old thread below that shows the US gauge sizes of batter cables. You will see a notation I put on the picture comparing the 300D with the insulation on it to the US size with the insulation on it.
I also found that by removing the terminal ends that the copper wire part of the cable was OK after soaking int in baking soda and water. OK means that the wires were not eaten away by corrosion. They just had a lot of the greenish blue with corrosion that copper gets. I either reused or replaced the ends and applied solder to wick inside of them and taped up the insulation.
What Gauge is a Stock 84 300D Battery Cable
Some people like to upgrade to US 10 gauge. But personally, after seeing that the 300D only has a 1/4-inch diameter wire bundle inside of it I don't think it is needed.
Autozone sells brass cable clamps and copper ends with an eye in them. You can crimp the down with a dull chisel and as I mentiond heat them and fill them with solder. That way they never corrode inside of the cable end.