I understand it can be done with a nickel rod.

In the 90s before we went with alum blocks on our blown drag motors , we would die grind , mesh and apply Titanium Putty to our lifter galley in order to keep our #2 and 6 from busting loose from spring pressure /cam lift.
I would try Titanium putty if possible as an alternative to welding,that is what some of the the Caterpillar guy's use on the large cracks or holes in their diesel blocks. Good for 16000 lbs per sq inch as well as high heat. Good luck!!

Where can I get some of this Titanium Putty? I have a spare 454 mark4 that has a crack. Thought that I would junk pile it. Maybe I will have it fixed and somebody could use it. Found that 454 mark4 4 bolts are hard to come by in my area and just hate to junk it.

Titanium putty is a high performance Epoxy , any commercial distributor should be able to order it.
I know there are various grades , some more resistant than others to heat,oil temps, etc. We paid $100.00 or so for a 1 lbs. container back in the 90s. I purchased ours locally,I don't remember our exact source. We still have a 7 year old spare short block that looks the same as the day we finished her, and that is after comming in constant contact with hot oil , fuel etc .

If you don't want to go the welding route and really are set on keeping your existing block, check out these people: www.castironrepair.com. They are in Buffalo, NY and I've been to their shop, pretty impressive what they can do. They repaired a cylinder head for me that was cast too thin under a valve spring seat and had broke through to the water jacket. It was about 1/16" wide and about 1 inch long. I didn't want to get a new set of heads as this was for a numbers matching car and this is why I went that route. The customer service is excellent too.

Cast iron can be welded using either Manganese, or Stainless steel rods. But before begining to weld the material it must be preheated to at least 400 degrees in the large surounding area to prevent further cracking.

Here is a link to AZ tools that is selling for $66.00. I remember a guy telling me he used this on the deck of a SBC and it held!? I can't verify his story. This said it only good to 350 F do you think there is enough of a safety margin there?
http://www.arizonatools.com/catalog/.../detail/16325/

and here was sone guys review of the product

"Have not used other brands, but I found the consistency of this putty to be quite firm, and hence easy to bridge a fairly large gap. It was straight forward to mix and apply, and appears to have bonded very well to an aluminum engine block that I was using it on."

I've seen freeze crack repair on blocks before. I had a freeze crack repaired on an old Delahaye engine. What they did was drill the ends of the cracks to keep them from running. Then the block was stripped and a thin slot was ground along the crack and the block was heated. After heating the crack was "silver soldered" .. This particular type of solder is more like "brazing" as it is gold in color... This was a repair in a water jacket. I don't know how structurally strong it was. But it worked for the water jacket.

As far as welding goes----Their is a procedure called SPRAY WELDING that a competent machine shop should know about. Basically it uses a special oxy/acetylene torch with a built in hopper that sprays cast powder onto the already prepped crack after the block has been oven pre-heated. The results are a very professional, and the welded area is now stronger than the rest of the unrepaired areas. This method can repair anything from valve seats, combustion chambers, cast camshafts and even cast crankshafts.
If you do a search their are a few more ways to repair a cracked block (other than welding) that I have posted.