Stalin took on the title of premier from 1941 onwards actually. He is also a full member of the Politburo.

But general secretary is the actually important position. It was from this position that Stalin rose to power in, and the position is the de facto leader of the party, thus the head of the real source of power in the Soviet Union (the party bureaucracy). Even though nominally speaking general secretaryship is not suppose to grant anywhere near the amount of power that Stalin had.

Titles and such wasn't terribly important during the Stalinist era anyway, between purging any independent political actor in the Soviet system and the terror, real power stems more from personal relationship with Stalin himself more than any other factor.