If your going to cool in sand, heat a piece of scrap metal and put in the sand while you weld the part. This will preheat the sand and not "shock" your welded piece. I have done this many times on cast iron and when I weld Stoody. Peening the weld with a needle gun or at least a small ball peen hammer greatly reduces the risk of cracking.
weldwell make a real good welding electrode for cast iron. it looks like shit while your welding but once you brush off the slag the weld looks amazing, ive used nickle with good results but aint to keen on brazing it, brazings good for plumbers! i class it in with the same catagory as using lead for body work.
I weld all of my cast iron cylinder heads, exhaust manifolds etc with Eutalloy powder (metal spraying), Done with the gas and the proper welding torch. Looks bloody good when its done, strong as, and best of all Ive never had a failure. Did a couple of mid 30's Packard heads a while back and I just did an exhaust manifold of a Toyota van a couple of days ago. Preheating and then cooling the item down slowly (bury it in sand) is the key..
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