it is a crap shoot welding cast...
i work in the NDT side of welding, which means, i test all the stuff they put out.
essentially, when you have a cast material(engine block), as opposed to the forged material (studs), they cool under different pressures, and are both formed differently, creating different melting points for the parent material. different grain structures also inhibits welding forged -> cast properly. it can be done, but my best guess would be not to do it at all.
it's brutal to say the least.
even the best welder, because of all the different zones of the weld, would have his bead brake within about 20 000 km just because of the vibrations an inline engine puts out. you would most likely see Heat Affected Zone (HAZ [1-15 mm from actual bead, both sides]) cracking and/or actual crater (stop/start) and longitudinal or transverse (straight up and down or across the bead) cracking depending on the weld method and type of consumables used. as illustrated in the picture below (bear with me =3). you may also find, that if welded and cooled incorrectly, the parent material will also crack and/or shear completely off... it's no good having a block that is broken. either get a professional shop to do it (Nat'l 4x4 may be able to recommend someone) or leave it be.

disreguard #'s 1-> 4,
5 is the actual weld puddle,
6 is the completed bead,
7 is the parent material