As herm0016 said, grind it out and weld from both sides.  Grind about 2/3 of the way thru from one side, weld it up then grind back about 2/3 from the opposite side.  This will assure that you get solid weld and get all the crack ground out.
A little preheat to 200F before welding would be advisable.  Make sure you grind all the way to the end of the crack.