I just braze stuff like that. I've brazed motorcycle cylinders and all sorts of cast iron parts. A person has to carefully preheat and post heat slow cool down but the temps for Brazing are such that the heat is lower and low enough that a person can do the job without cracking it any further. A person may have to stop drill the ends but it depends on how rough the casting is on each end of the slot mentioned. Careful profiling can be done with a 1/8" or 1/4"carbide burr and a die grinder to smooth it some without removing too much metal if the slot is large enough. If not then just smooth it best you can with sand paper or a small hand held grit blast device using glass beads. Clean it well and clean again. Use a good flux and quality brazing rod.
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