Here's why mig fails on so many cast iron parts, and high-nickel filler does not.
When you heat steel up to red-hot or higher temperatures, it undergoes phase change. Not just solid to liquid, but also bainite, martensite, and some others. These phases are basically re-arragement of the atomic structure and covered by this horrid little diagram.
View attachment 1715401417
You can see this in real life when you heat up a steel wire strung between two points. It'll droop. Remove the heat and as it cools it'll shrink, and tighten back up, then droop some more (phase change), and then tighten back up again.
Why all the metallurgy?
The problem is that this phase change occurs in the weld bead when you lay a steel weld bead in cast iron, which doesn't experience the same thing. Since they're expanding and contracting at different rates, re-cracking is highly likely to occur and it usually cracks right next to the weld, as soon as it cools. Especially with low-grade cast iron or iron with lots of graphite in it.
High-nickel content filler metals (nickel brazing, too) don't experience the phase change, and so the chances of cracking are FAR less.
Being that is a low stress part with little carbon, I think brazing would work but the ear will see a lot of pushing and pulling as the manifold heats and cools. The other option is that you can also use stainless fillers with a TIG (or MIG) welder. I've used this to a great degree of success in the past, and it's super easy to do. It lays in very easily. When I first heard this I thought 'BS', but a guy I worked with broke his antique cast-iron Ford wrench which sees a LOT of force. I tig'd with 316 filler and he's using it to this day.
Any welding shop that does TIG will have stainless filler since that's the preferred method for stainless work.
Edit: this is just how I understand it, and the odds of the understanding....pretty slim.
But you can use stainless to weld cast.
