China OTL didn't detonate it's first nuclear weapon until 1964, and didn't deploy missiles capable of reliably reaching Moscow until the late 70s: even if the Soviets don't immediately take a NO NUKES FOR YOU hard line, the Chinese will probably need a couple decades to be in a position to really wreck European Russia. (Don't know how much Chinese technological progress will be retarded when cut off from any source of tech knowledge they can buy, borrow, or steal other than the Warsaw Pact itself).
IRL they got the DF-3A, their first missile that could hit al of russia, in 1971, although it took a few years to make it into an actual force. Before, their DF-2 (~1964 operationally) (which they used for nuclear tests) could hit most of the central asian states as well as many siberian cities, like novossibirsk or Omsk, although not the other industrial cities near the ural.
If it's a 1960 ISOT they have at least 15 years before china can hit european russia and damage it enough.