Recently repaired cast iron barrels using the M.I.G process (mild steel wire) argomix gas , "Brian" set Me right ..came out a treat .
Put some heat into the item in the oven , stitch carefully after cleaning and prepping edges with a grinder ..dont forget to angle the broken fin down a fraction to allow for distortion bringing flat then allow to cool down in the oven , trim with grinder later .
Hope this helps ?
Steve ...