Welding even with a cast iron rod is tough on old castings. I've done a "stitch repair" with success. Drill, tap and screw in a repair plug with sealer on it, the next one catches 1/3 of the dia. of the previous plug and so on. Time consuming but a successful job. Old cast has so many impurities in the metal it's hard to run a good bead, then there is the heat issue. When using a sealer in the coolant, I use nothing but Bars Leak, it won't plug anything else other than the leak. Good luck.
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