From everything I've read on repairing cast iron cylinder heads, the preferred method is to preheat the head very hot(over 600F), then use a spray welder to fill in the ground out crack, then use a stepped heat down process to prevent crack formation..
Of course, this requirea A)a spray welder(not cheap) and B) a large industrial oven with a step down controller(seriously not cheap)
The reason why it cracks, it because the weld material is much hotter than the base material, causing a "cold sink" and contraction. This is why Oxy/Ac is preferable to Electric welding because it heats up the base material as well.
There's an old issue of SS&DI that has an article about a Cylinder head shop that specializes in the repair of $$$$ heads like Hemi heads(this was a coupla years before the repro heads came out) Ford "Cammer" heads, etc... I'll see if I can find it and maybe scan the article.
