Next you need to decide where to tap- I choose the drivers manifold @ the casting button, convient location and by several accounts one of the best places to measure from (mine is grey because it's been jot hoat coated )
You need to use a drill press IMO and you should clamp the manifold in good- I used several varying wood blocks to accomplish this. I started w/ a 1/8th " drill bit and then stepped up a 1/6th" at a time till the final drill 8.4mm. I used a good "cutting oil" throughout. Go slow w/ steady pressure.