My understanding is the cases are close to 356 alloy. I am told that you can weld in plates of 6061. As to the rod--
http://www.esabna.com/us/en/education/b ... -alloy.cfmI am sure the experts here will have specific advice. I have never heard of people useing wire-feed MIG, its always TIG.
One thing I do know is you should pre-heat the case before welding on it. Otherwise the case half is sure to bow, and then you have to deck it, and then you start running the flywheels in the wrong position. Kenny Puccio, one of the best welders in San Jose, had a "shade tree" method for this. He would get his oxyacetylene torch and and run pure acetylene, and play it over the case half until it was covered with soot. Then he would set the case half on top of his wood-burning stove. When the soot all burned off, he got out the TIG and did his welds. He did not tell me if he kept it on the stove while he welded but I suspect he did. He then let the wood fire burn out, and the the whole mess would cool off with the thermal mass of the stove plus the cases. I think he told me that bolting up the halves does no good, the idea is to not have any residual stress in the case, not to hold it flat while it cools too quickly and gets full of stress. If you post-heat treat I believe it will take out the stress, but you will still have a bowed case half.
Of course, all this is predicated on how much welding you are doing. If you are filling in a 1/4 inch dot, you might just hit it with the TIG, lets see what the gang says. You do have to make sure the mating surfaces are dead flat when you are done. Feeler gauge and surface plate or a thick mirror or piece of glass works OK. If its bowed, my machinist buddies all had a surface plate with sandpaper glued to it, to get things dead flat-- use a figure-8 motion. Getting the sandpaper glued flat is an art unto itself.