Age old question...Bad Battery or Bad Alternator???

I'm not sure "the best way." You might be able to get by with a larger body bolt. These are sort of "self tapping" Some are tapered, some have a split part way almost like a tap. If you cannot do anything else, bend up part of the flange so you can "get to it" and drill and put something like a 10-32 screw and nut through there, with a pigtail wire. Run that to a known tight good ground.

By the way, if you run Mopar ECU (ignition) that box is another MUST ground item.