I've fluxed a lot of cast to galv.
Toyota manifolds are almost always hunks of cast iron & I've cut several up & welded galvanized mild steel from Lowes be it EMT Condiut, or like... The 2 1/4 - 2 1/2" * 10' to 15' sections.
The big trick is to have your *** outside in a 100% fresh air supply. Inside with a fan isn't gonna cut it. The first time your head get's anywhere near those gases you're going to choke & cough. & then about 4-8 hours later, you're going to feel like you have a hangover. It's happened to me like twice now & I was outside in the driveway.


& what those experts are saying is right. It's not extremely strong. I honestly wouldn't trust the welds on a manifold, or exaust piping to hold up a turbo of any size. I find it hard to weld the 3/8-1/2" cast to say 1/8" gavanized pipe. If I don't keep it moving I burn through. Then again, I'm not exactly god's gift to welding... But I am to grinding! ROTFLMAO!

 

You mean weld. Sprinklerfitters screw, pipefitters weld, but you knew that, didn't you