Are you referring to 1945 where ROC became a member of permanent security council or 1971 where PRC become a member of permanent security council? Because by 1971, PRC is a nuclear state with ability to produce its own tanks, fighters and submarines. More importantly, PRC also managed to defeat a UN force at Korean Peninsula, Indians (which is, at the time, believed to be the leader of third world countries) during the skirmish at 1962 and managed a stalemate with USSR in 1969. Basically, PRC managed to stood its ground against all other members of UN permanent security council in open conflict and earned recognition for its military strength.
Now, RoC's track record isn't as great as PRC. It was able to wear out Japan in a war of attrition. Its expeditionary force's performance in Southeast Asia was also recognized by Allies. I will admit that the strength between China and India is rather close back in 1945. In fact, if history is different, there is a very real possibility for India to get on the permanent council. However, the reality is that when UN permanent security council is founded, India is still a British colony while China is an independent nation and by the time India managed enough influence, 1962 occurred and effectively confined India influence into the south Asia subcontinent.
