Pre-turbo
Take your time when drilling the whole and perform the "step up" method wherein you start with a small hole (1/4inch) and then step up through the drill bits till you get to the desired size hole (i think it was an 11/32inch). Vacuum out the manifold when complete. Then take your time and tap it out using bearing grease as a catch for all of the shavings VS allowing those to settle in teh manifold and then go through the turbo at first start up...
The key here is taking your time... Keep in mind, the old saying - "measure twice - cut once"