Before June of 1940, the Stalin of our time line seems to have overestimated the power of both Britain and France. Indeed, he seems to have based his plans on the assumption that the war between the Anglo-French alliance and Germany would continue through the summer of 1942.
Had France and Britain promised to defend Poland à tous azimuths, and this failed to deter Stalin, the French and the British could have done little more than attack peripheral areas of the USSR. As the most vulnerable spot in the USSR would have been the Pacific littoral, the French and the British would have had to take the attitude of Japan into consideration. That is, they might have had to trade support for the Japanese in China in order to assure themselves a free hand in the vicinity of Vladivostok. This, in turn, would have led to a situation in which the Chinese Nationalists sought support from Germany and the USSR.
Close cooperation between the Anglo-French alliance and Japan would not have pleased the United States, which was, at that time, both pro-Chinese and anti-Japanese. Thus, it would have been harder for President Roosevelt to provide aid to France and Britain and harder still for him to wage an undeclared naval war against Germany.
If the Anglo-French (and, perhaps, Anglo-French-Japanese) operations against the Soviets proved successful, the USSR would have deployed substantial forces to the east, both to regain what was lost and punish the Japanese. While this was going on, the Germans would have been preparing for their invasion of the Low Countries.
The proportion of the Soviet armed forces that Stalin was willing to deploy to the Far East would have depended upon the strength of his belief that a German invasion of the West would have resulted in a return to trench warfare. This, in turn, would be a function of the performance of British and French forces in the Pacific region. If the British and French performed well, Stalin would have been confident of a long war in France and Flanders and thus willing to send more forces to the Far East. If, however, the British and the French performed poorly, then he would have retained a greater proportion of his forces along his western border, to keep watch on his German ally.