well you only need to grind off the head on one side of the rivet...
Then you want to drill more or less on centre, but not critical with a small drill bit all the way thru, put in a larger bit, drill all the way thru...keep going...
once you get to a drill bit that is JUST breaking thru one side of the rivet and into the metal of the ball joint housing... you can stop drilling.
Then mount the arm in a vice, get a good drift and a good hammer, give it a few good whacks and the rivet will drive out...
then you need to get a grinder with a thin blade, and cut thru several small tack welds put there by the factory..
once you've done that , a couple of light taps from the hammer and the ball joint will twist out of the arm.
Now Fiat never supplied the ball joint as a replacement part... probably something to do with public liability should any sort of fastener apart from a hot compression rivet be used to re attach the ball joint to the arm...
having said that, I've found that an appropriate sized high tensile bolt and nyloc nut seems to do the job quite well....if you want to get fussy you can shave the head of the bolt thickness down a fraction so it ends up no taller than the original compression rivet.
SteveC