I would caution you about the Home Depot “welding cable” - Not that it is bad, just stiff. They tend to sell “flexible” cable that has quite large strands in it, making it way less flexible than I might like. The cable used in mid to higher welders is always made up of finer strands that are way more flexible but at a big price premium.
Now, the car’s electrical system really doesn’t care how flexible the cables are, just that they are robust (thick) enough to transmit the amount of current you’ll need to crank over the starter. 4 gauge is good, 2 gauge is better (but more expensive).
Go have a look at the Home Depot stuff - pull out a length of 3 feet or so and see what you can get for a tight bend radius and ask, will it lend itself to where you want to run it? Will it fit along the contours of the floor, especially at the back where it curves along the floor and then up through the bulkhead?
gn