Centre punch the bolt heads then drill out, if you use a drill that is just bigger than the bolts(thread) diametre it will just take off the "cap" or top of the bolt head.
The first drills do not need to be deep say 4/5mm, and are just to centre subsequent drills that's all, I've no doubt that the actual threads are not tight, but you can easily get grips/etc on if they are.
tabetha