The soviets made their first undertakings to attack Japan at the Teheran Conference in 1943, well before any deliveries of the p-63.
Stalin promised to enter the war in the Pacific after the German defeat in Europe in return for the Allies ' promise to open the Second Front. The allies were enthusiastic for Soviet support. The chinese had proven to be incapable of dealing with the massive Japanese army and the allies simply lacked the strength to tackle the IJA head on. The "island hopping" strategy was working, and the allies knew that the strangulation of Japan could work, in a military sense against any rational opponent, but the Japanese were anything but rational opponents. Their willingness to die to the last man for the emperor, made them a formidable and intractable opponent. At the time of Tehran, no-one could rely on the Chinese, no one could rely on the b-29, no-one could rely on the atomic bomb. The allies were aware of Japanese plans to initiate Kamikaze attacks, and the expectation was at that time that this would inflict heavy losses on the Allied fleets. The allies were just not in a position to start dictating about how the Soviets might use one type of aircraft being sent to them, es[ecioally when the Soviets were getting so much out of it