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Steve, I make the pieces that I use for repair out of cast iron , if you use steel you will have the same problem that you have with nickel rod . The rate of expansion of most anything but cast is so much greater than cast that most times the job will crack before it is cool. On cast iron , use o/a torch ,cast iron rod with proper flux, and much preheat . It's best to build an oven ( stack fire brick around to form an oven ) leaving ports thru which you can apply heat with large propane torch as in weed burner .Heat to red glow then weld and you may have to reheat many times as you weld to keep the temp near 1200 deg F , do a post heat of near 1500 and hold for 15 minutes then close up the ports in the oven cover with more brick and finally cover the brick with asbestos blanket. Do it late at nite , go home when done , uncover it in the morning as it should be cool by then. And yes those are jobs in my shop , I do quite a lot of cast repair , much of which is rusted out or burned out manifolds. Broken ones are much easier to fix. george
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