@TheDesertFox Although this topic is quite old, I strongly disagree with you. In every game, Japan pushes through China AND the Far east to take IPCs off Russia. Yes, they are only worth 1 IPC each, but it sums up. Russia usually loses 4 territories here, which not only makes them lose 4 IPC (which is more than 10% of their starting income) but also grants Japan 4 additional IPCs 8which is more than 20% of their starting income. Plus, you usually have many fighters nearby so Japan can crush the Russian infantry pretty easily there. The Russians have no offensive capabilities there whatsoever (unless they send something there but believe me, they need it against Germany rather).
If you want to implement a Manchurian Pact there though, I suggest using a rule from revised: There is peace between the Soviets and Japan at the start of the game, however, you can declare war at any time. When you do so, the Player that has been delcared war against receives 3 infantry units that he can place immediately in any territory that is under attack by the opponent.