I'm gonna take a blind azz stab at this for now without looking it up because I think this could be very good thread if we get enough folks runnin with it, although there are some companies out there specializing in overlay applications like this-Cor-Met I know off hand I've dealt with has very knowledgeable people.
It seems to me that due to the high temp application your gonna need some cobalt alloyed filler (as opposed to a Cr carbide, though I could be mistaken-we used a lot of cobalt based hardfacing alloys for extrusion dies at a pipe mill I worked at, and it doesn't crack as readily) and since its cast iron you're working on(of which I will admit I know doodly about) your gonna need high nickel (which can also handle high temps well, depending upon the alloy) to help prevent cracking. 
And to hopefully insight discussion on cast iron correct me if I'm wrong but given a spheroidized microstructure wherein much of the carbon is precipitated as carbides, and that the microstructural matrix is pearlitic I would venture a guess, a guess, that the material is not as crack prone as one might expect.