Both the positive and negative battery cables need to be at least 6 gauge, preferably 4 gauge or larger, especially the positive since it has a longer run.
On the front of the block there are bosses drilled and tapped and the bottom one is a good place to hook up the ground. Clean it to bare metal and use a good washer and lock washer. The block needs to be grounded to the frame and yes the block should be grounded to the firewall.