Either mount someplace you can use "through" bolts, nuts and star locks, tack weld nuts behind, or solder a ground wire pigtail to the case and run that to a good ground.
Example, if you decide where to mount, pull it off and scrape a little paint off the case on the "under" side of the box, use a "big" iron / gun and solder a no14 pigtail right to the case.