I wouldn't reccomend it. There are shops which specialize in the repair of blocks, but it might even be cheaper to find another block.

My supervisor tried welding the block on his tractor, and each weld created a new crack further up as it cooled.

If you are REALLY good welder, and have the capability to preheat the block, anneal it after weld, and cool it SLOWLY, then you might have a chance. Otherwise, send it out.