I did it slow and carefully. I had no problems. As a matter of fact, I used (I think it was) tungten drill bit and it went through the frame like butter.
I would measure 10x and drill once. I used a hydrauallic (sp) jack to hold the arm in place, set it correctly, measured at least 20x...hey, I'm excessive compulsive..
and then I drilled into it.
BTW, I also added some silicone caulk to the inner brackets, the bolts and the tip of the bolts. I figured if I did that, not only would it seal fairly well, but also the caulk on the flat tip of the bolt would also help cushion any wire looms that might come into contact with the bolt on the inside of the 'A' frame.
All good for over 1000 miles so far!