I apologize for dredging up an old thread but I just ran into this exact same issue. When I discovered that the nuts provided with my running boards were getting tough to fully tighten onto the factory studs, I figured I'd just force 'em with an impact driver. Whoops - I snapped one of the studs clean off. I didn't even think to chase the studs' threads first. Curiously enough, I was able to tighten all of the other nuts with hand tools by spraying a little white lithium grease on the nuts before installing them.
As suggested in this thread, I drilled out the snapped-off stud and re-tapped the hole with an M7 tap I already had. However, because the plate that I was threading is so close to the body panel right behind it, I couldn't get past the tapered part of my tap, so the holes weren't threaded all the way through. I didn't realize this until I ripped the threads off of two bolts that I tried to install. I ended up cutting the tapered portion of my M7 tap off so that I could get threads tapped through the full depth of the hole. After that, the whole process went pretty smoothly, but what should've been a 15 minute job (if I'd RTFM) turned into a multi-hour ordeal.
Anyway, I just wanted to say thanks to the contributors in this forum thread! I was just going to go with just two fasteners on one of the brackets on my running boards; it hadn't even occurred to me to try to tap a new hole because I figured there wasn't enough material there to tap threads into. Here's what mine ended up looking like:
I was lucky to find a thick 1/4" x 1" fender washer at Lowe's that covered up the hole in the mounting bracket completely.