I was try to hold of my DC man, but he is on a cruise to Alaska at the present time. We use a lot cables larger than a 40 and he quite familiar with the use parameters of same.
You can always double the cables. Keep them as close to the same length as possible. As you have stated it's the total length of both +/- that is considered.
Just curious how are you going to get the 800 or so amps needed?