Hey Cal,
I am not a fan of brazing cast iron. It is better if you can find an engine reman shop or possibly a welding shop that can nickle weld it. I was going to write an article on this, but my buddy closed his engine shop and retired. The big problem with welding cast iron is in cooling, it must be done very slowly. It takes me 2 or 3 days to cool a cast iron repair, then just maybe it won't crack somewhere else. As to a cold weld product, there was something called 'Pyroputty' I don't know if it is still available, or where to look for it. JB weld won't take the heat. There are repro manifolds available for your IXB-3 engine, and they are reasonable in price.
Take care!!!
Andy