I've TIG welded cast iron once, using specialty rods. I got away without preheating. The trick there was to put down some weld, and then immediately grab a hammer and punch and start beating on the weld bead as it cools. Then repeat as needed until done.
When you weld cast iron, the weld shrinks enough to pull away from the base metal, which is why you normally preheat the entire workpiece. But by peening the metal as it cools, you're forcing it into the gap that's trying to open.
I agree with others, a new block is probably better. But I thought you might want to know this trick in case it ever comes up.
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