It is cast as stated above. The tricks, if you can call it that is to make sure the manifold is clean. Drill a small hole at the very end of the crack, this will help prevent the crack from growing when heat is applied to it. Bevel the crack forming a V the entire length. Preheat it, a propane torch will work, also it doesn't leave deposits. There are numerous rods for cast rods, a general repair rod should work best. Call a welding supply and they will help you pick the right one. I have used the 1/16" before, smaller rods work better for this type of work. They take less amps/heat and are easier to control. Lastly post-heat. This is where most fail and new stress cracks form. You can find more info on the net about pre and post heating. I would NOT try to weld it with a standard mig unit with standard wire. A TIG unit would work if you get the right fill rods. If you get the heat (amps) on your welder to high the casting will slag and deform. 50-60 with a small rod should be your starting point. See if you can find an old exhaust manifold to practice on first to get your settings.
Sorry for going a little long there,
Good luck with it,
JR
