Swarfy there is no need to enlarge the hole to accommodate the bearing, you simply have to Taper the inner shoulder in the head stem to allow the Inner bearing to protrude further into the tube. The outer cup still sits on the full width of the internal shoulder. Hope that makes sense? I don't think there is any weakening of the head stem.
I learnt this lesson the hard way as I had inserted the outer cups before I knew of this issue. Fortunately I was able to fix the problem with a die grinder without damaging the new cups.