Find an old school welder who knows how to work with cast iron. The stuff is brittle, and very unforgiving of mistakes. I've been more successful using bronze fill rod and flux to work with cast iron instead of nickel alloy arc welding rod. Preheat, weld a short section, peen, and post heat the weld, plus you'll need to bury the part in sand and let it cool down for several hours after each welding session. "Rinse and repeat"! It's a good idea to keep the manifold bolted to a junk cylinder head while you're welding, to minimize warping and heat stress. The head acts as a heat sink, as well as reinforcement. Do not try to do the entire job in one session. In addition to adding a second outlet flange, it's also necessary to have a baffle in the center of the exhaust manifold that almost (but not quite) blocks off the back of the manifold from the front two ports, just behind the heat riser area.
Jerry
