Welding cast iron is a bitch, welding old cast iron can be hell and very unpredictable. There are a couple reasons why most welders will even tell you to forget it... I just can't remember the reasons. :P It's more a matter of penetration and inconsistent density. It can definitely be welded (I suppose that's up for argument but it's not my argument to make), but the couple people in that industry with whom I've spoken on the matter have all said that it's a waste of money and effort.
You'll usually have a horrible time finding a professional welder who will weld a cast iron manifold; Those are small and require less precision. It's not unlike when you crack the exhaust side of a turbo... it's trash.
If you can find someone who's willing to weld it the odds are that after the welding and all the milling and boring and flushing that it would still be cheaper to buy a used block and only do a mild rebuild.
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