Welding cast iron isn't that difficult. Use 99% nickel rod. Preheating is better but I have done it without. Dirty cast iron seems best welded with a stick welder. Clean can be welded with tig. It isn't enough to shine it up, grind the crack down to clean metal. You will have to grind a lot as the rusty and contaminated layer is thick on an exhaust manifold. Vee the parts at the weld but don't vee to deep as the iron burns through easily. I have found that you need to keep the arc very short. It isn't at all like welding mild steel.

Jason