The big issue with welding a head or block is that the grain structure of castings is different than that of rolled materials. To be properly performed, the item (block, head, etc) needs to be heated to a high temp before welding. Then to preserve the tensile strength it will need to be heat treated after welding. This is sometimes refered to as a "foundry weld". NOTE: almost all castings have some welding performed during thier manufacture.
If properly performed the results will be equal to that of the original casting. However it may be expensive.
If this is not performed properly additional cracking may be experienced in the future.
