By bonding the box to your EGC, you are bonding it to earth. Remember that your EGC is connected to your Grounding Electrode at your service through your main bonding jumper. So, by bonding your box to the EGC, you are also tying it to your Grounding Electrode system.
The method you are planning on using should be fine. Don't forget to scrape the paint off under your lug to ensure a good connection! Sometimes I will use a small bus bar kit meant for a panel when I have a couple larger wires coming in. Don't have to use a split bolt, and you use less pieces = less labor.
mike
