I've stick welded cast iron with the Ni-rod available at most welding supply vendors. I have also used GTAW and inconel tig wire with decent results on manifolds. The Ni stick rods can also be used to tig weld cast, if you clean the flux off and sand or scotch brite the rod. Pre- heating and letting the weld cool slowly is a must to prevent cracking. You also need to peen the weld with the round head of a ball peen hammer while it cools slowly to releive stress. Keep a torch or rosebud going and heat the weld lightly while peening. It is good to have an extra pair of hands or two to help with this. I would weld a couple of inches at a time and as I pull off of the weld, the helper will start peening up and down the weld joint. Not too hard! The torch needs to be keep things warmed up as you go. There are probably more scientific explanations and techniques, But I have used this method on many occasions with success. You have to have a high nickel content filler material. Good luck!