Actually, the USSR stands a good chance of getting off scot-free in regards to the nuclear exchange. At this stage, the Chinese have only slow, vulnerable bombers with with to deliver their weapons, and the combined PVO and VVS would down them easily.
After taking Manchuria, Inner Mongolia, and Xinjiang, the Soviets can easily make use of the ravages of the Cultural Revolution to set up stable client states. They can just pick and choose from local cadres who will jump at the opportunity to do their jobs without (as much of) the endless political struggle sessions, shaming, and general Maoist insanity. Unlike the Japanese, the Chinese will soon see that the Russians are still "socialist brothers" and be more open to collaboration. And whatever the barbaric actions of individual Soviet soldiers, their commanders are unlikely to set up their versions of Unit 731, commit outright Nanjings, or turn women in to crack whores.