I drilled and tapped my RevHard manifold about 1" from the flange on cylinder 4, it doesn't really matter which cylinder you pick. I did this 3 years ago, no problems yet. The shavings are so minor, they will blow right through the turbine, I coated the tap with grease so most of them stuck to the tap anyways. Don't tap too deep with the NPT tap, this way you won't have to bury the fitting.

If you want to see pics, just click on the link on my sig, the pic is in there..good luck.