It was the USSR and the US who were the biggest allies in WW2. China was a non factor. And after WW2, the US opposed Mao by supporting the losing side. The USSR supported Mao's revolution. Trade began between China and the US in the 70's but it was not part of a plan to defeat the soviet union. It only really picked up at the end of the cold war.
I don't think Japan is being usurped. If and when the re-unification happens, it will be chaos. South Korea just hopes it does not bankrupt them. Having a new source of inexpensive labor will not help Korea surpass Japan. China is close by and there is still plenty of inexpensive labor to be had there.
Also, its not like just Japan uses Chinese manufacturing. The whole world does including the US. Japans engineering head-start was not given to them by the Marshall plan(which was mostly directed at europe). They earned it starting in the 50's.