I (well my welder anyways) has been successful stick welding cast iron exhaust manifolds. Here is the rod he uses:

http://www.alruqee.net/ind_mkt/RIM-CD&WEB/welding_consumable-files/utp/265-85fn.pdf

Other than the rod to my knowledge the secret is:

1. Only light preheat to take the sweat away.
2. Run short beads to keep the heat down.
3. Aggressively peen the weld and surrounding area after each pass with a blunt air hammer.
4. Get it in the sand, kitty litter, or asbestos blanket right away as grinding will generate “wind” that can induce cracking and leave it there for several hours to slow cool.

This process has worked repeatedly and passes MPI every time for me. I have no experience with brazing these however Stuart knows his stuff and I’m sure his suggestion will work as well. I also would not try to weld the tab back on but build a new one with the rod and a flat pc.of copper against the flange surface would give a good surface to build upon.

Good luck,

Curt B

1952 1300 Canadian 1/2 ton restomod
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