I'm just doing this at the moment. I don't have room for a 2wd manifold and need to do the wastegate piping myself. I'm going to mill a hole in the right place at the angle i need and then insert a mild steel tube. I'll tack it in place with a mig and mild steel wire and then take it to a welding shop who specialise in cast iron and get them to do it. From what I've read it needs pre heating and then cooling very slowly to stop it cracking. Mild steel should flex enough in use to stop it cracking, stainless would most likely crack. As the whole lot will be going to Zircotec it should be nice and rust proof when its ceramic coated.