Thanks everyone! I’m going to weld it shut, after drilling the end studs.
Unless you're WAY more skilled than I am, welding is not the way to repair cast iron. It needs to be preheated and cooled slowly after welding. You may even need to re-machine if it warps on you.
It's just a water jacket, so epoxy or Jerry's drill-tap-plug method would be way better.
I tried brazing a cast iron engine side plate once and wound up tossing it, as it was warped so badly that there was no chance of getting it to reseal again.
