Don't forget about a grommet or a cable entry bushing when routing the cable in through the firewall. Sometimes, it's just better to have the right stuff. You could use welding cable, but the jacket isn't really designed for continuous contact with fuels / oils etc. Battery cable has crosslinked insulation that is rated for high temperatures, as well as fuel / oil resistant which is a good thing, you can buy it in bulk.
Most car audio specific kits contain CCAW ( copper clad aluminum ) which isn't necessarily a bad thing, as long as the ends are well sealed in the engine compartment area, there is no issue, as well as it is lighter and less expensive than copper. The downside is that the cable will need to be a bigger gauge vs. copper for the same draw.