More Russians in Kazakhstan

Just have things go in the Soviet Union as they did IOTL, simply more effectively as far as Russification is concerned. A more successful and extensive Virgin Lands campaign (probably one that involves a more diverse, less exploitative kind of farming and thus doesn't wreck the soil so fast) that lasts longer is going to attract more Russians and other Slavs to the area. The Soviet Union never breaking up helps as well. But if you look at a demographic map of Kazakhstan today (if you haven't already), well, we're already halfway to completing your challenge - depending on what you mean by "northern half" anyway.