Re: Cracked Exhaust Manifold
A castiron piece can be more easily welded on a corner or edge than across the middle or interior of the part. Probably the most important thing is to bolt the part and the broken off ear down to something heavy and flat after V'ing out the crack. Then weld with a cast repair rod for only short distances at a time and quickly peen the weld to relieve as much of the stresses as possible.
A standard recommendation to repair cast iron by welding is to heat the whole part to above 1200 degrees F before welding, make the weld above that temperature, and then allow the part to cool over many hours.
For an ear like you have, I doubt this will be necessary, but you may want to allow it to completely cool between welds if you don't get it done with the first bead to minimize cracking.
If the resulting flange isn't completely flat, you may be able to carefully file or more likely grind it flat or even take the manifold to a shop and have it surfaced across the whole face to make sure it seals well. The cast iron next to the weld can be very hard after the welding.
I have welded in missing parts of a Ford diesel exhaust manifold with a small mig welder using standard ER70S-6 solid wire. the repairs were the "female" ends of a multiple piece manifold so they were sort of on the end of the part and had some room for expansion and contraction from the weld and normal heating and cooling cycles. Not pretty, sort of like a drunken mud dauber nest, but it keeps the exhaust in so far.