Your going to find drilling through weld, into a hardened suspension bolt to be nearly impossible without fairly substantial tools (speed controlled drill, lots of good quality bits with an understanding how to sharpen them etc). Even well prepared, the odds are your going to get a bit sideways and chew up the arm, and or the body.
On a centered mill, I wind up crooked now and again. Doing it upside down under a car? Miserable for sure.
I wouldn't start this project without having backup transportation.
There's a pretty good chance what's happend is someone's got the bolt stuck going in, or out, and snapped the head of the bolt off, and filled whatever hole remained with weld. Those bolts in particular tend to get somewhat eaten by rust in the shank area. It's probably not removable even once the arms out of the way.
The weld could be 50 thou thick, or 2" of poured in rod/wire. It's also really not a terribly safe situation as it stands.
Under the arm, behind the 'hole' in the body is a semi captured nut. As there is a good chance there's a entirely un-repairable bolt stuck in, your options are probably going to wind up cutting a hole in the body to get in behind that nut so you can remove, and replace it.
And if your going that far, it might be easier just to slip a cut-off wheel between the arm and body to try and clip the bolt entirely and shift the effort into replacing that nut instead of trying to drill the weld off.
Miserable luck, anti-seize any remaining hardware once it's fixed so hopefully it won't happen in the future
Paul