Honestly I can not recall what I used to remove the bung holes on mine - only that it was a non-issue. I think I used a vice grip and just opened it up and turned it - the bung hole is threaded. That said, I might have used a screwdriver to do the same thing.
I then screwed in a close nipple of the same thread for a short smoke stack.
Mine was well oiled (coconut oil barrel) and did not take a huge amount of torque to open.