i am far from a professional at welding but i have done some reading up on it. welding cast is a differant beast, part of the problem is that there are so many types and grades of cast that its hard to make a one size fits all fix.
in general though welding cast with steel wire on a mig is asking for trouble, can it be done, yes it but should it probubly not. there are two things working against you, first the metal weld cools at a differant rate than the cast creating tons of micro fractures that can pop wide open as it cool. this can be negated somewhat by peaning the crap out of the weld as it cools and dramatically slowing down the cool time by covering it in sand. its still a toss up if it will work. the second is that the mild steel wire the mig is putting out can pull the carbon out of the cast and it makes the weld super hard martensite that is very prone to shattering and alnost impossible to drill or machine.
some have had some success welding cast with SS wire since it greatly reduces the carbon problem
cast iron seems to be best welded with pre heat, chamfering the weld area and a stick welder with a high nickel content rod. although i have heard of people having good success using Royal 44-30 cast iron mig wire even patching up cracked and holes engine blocks. its $$$ but seems to work, i have not used it myself though so try it at your own risk.