My guess is that the Japanese attack on Russia will be prevented in the most logical and historical ways:
First: Strategically, the Soviet far east is of little value. With the new territories, it will be a long, expensive way to Moscow if you try to go the Siberia route. Which was the historical reality. Plus, remember that Japan starts with only 27 IPCs in 1940, and the US will be at 57 in the Pacific and perhaps over 100 in the global game. Most of the economic gains that Japan stands to make are in south Asia (as IL said). Plus you can throw all of those troops at Russia, and still have China, India and ANZAC breathing down your neck. Not to even mention the USA.
Seems to me if I’m playing Japan in the Global game, I’m going after the cash as well as the nations that stand a chance at actually doing damage to my economy. Sure, it would be nice to help Germany take Moscow, as in every other game, but it seems to me the longer distance and relatively tiny IPC value of the territory will keep Japan occupied elsewhere. I still think the rules should allow Japan the option to attack if they wish, just as USSR should have the option.
Furthermore, and from the historical perspective, (for K-Steel) Japan fought quite a few skirmishes with the USSR on the Manchurian border before they signed the pact. And the Japanese got their asses kicked every time. In at least 3 separate attempts the Japanese were prevented from advancing into Russia with very little effort from the Russians. General Zhukov was in personal command of the forces at the time and they had little trouble with the Japanese. This is why Japan signed the non aggression treaty, not because Russia was afraid of Japan but because Japan was afraid of Russia…
Hence, I anticipate a pretty hefty force spread out about the Far East. I think the placement of extra troops there in revised and AA50 was one of the best improvements. I could easily see 4 or 5 in the territory directly north of Manchuria and at least 2 spread out in every other space (remember there are now 3 other territories bordering that one: 9 infantry would be a pretty tough fight). Even if Russia wasn’t ready to fight a modern war in 1940, they sure had a lot of people kicking around the country.
Plus there’s the separate victory condition thing…
In any case, I think the 1940 start date as well as an IC in India and Australia will keep Japan busy in the south Pacific more than in any other game to date.