Cast iron behaves very differently than steel when welding. There are a lot of camps on what is best for repairing it. Most likely if you do MIG weld it, it will crack again, either in the same spot, or near the repair.

Generally the old way to weld cast was with an arc welder and high nickel-content rods. You preheated the cast first and then welded it and allowed it to cool by itself. This was OK, but, it wasn't exact science as you didn't really know how hot you were preheating to, and couldn't maintain an constant temperature.

TIG is the best method now for cast welding. It keeps the heat concentrated at the area of repair. You can also choose exactly the correct filler rod for the job, although what was used to cast your manifold is anyone's guess.

Replacing it would be your best option overall.....