I have to disagree. It is possible to weld to cast iron, it just requires proper technique. See this page for a bit of info on the subject: Guidelines for Welding Cast Iron | Lincoln Electric

Two of my friends have stock cast iron manifolds with wastegate flanges welded to them. One of them was on the car for several years with no problems, not sure where it was done, though. The other one was just done at Coastal Dyno. It's held up well so far, the weld looks good and the price and customer service were great. I'd give them a call.