Theoretically, a Soviet victory is possible right up until 1989. But in terms of it being probable... ~1983.
during the early 70's it might not be so ..
the real thing is simple. no way any war would happen with out nukes. No one could attempt to start with out using them, or finish with out them.
I don't see any way once tanks roll in either direction that someone doesn't press the button, especially the longer the fighting drags on.
If either side and in this case, if the soviets started it. great they get victories for the first weeks. they cross the rhine. they get nuked. there is simply no way this stays conventional. same the other way. Nato forces get past Smolensk and its press the button time ( if it didn't happen after the polish boarder )
now even if you hand wave away nukes. the soviet navy is toast in almost any decade, but assuredly in the 80's. air power wise, the Nato isn't that far off. numbers are not everything and in this case when push comes to shove, Nato would win out. especially in anything after 1982 or pre 72 . that window is short .. but even then, I question the ability of the soviets while on paper quite strong, but in reality made up of players that don't want the soviet yoke anyways. its not like the USA forced france to do anything or Germany or others. they were partners to the extent that they wanted to be.
quick edit.. for the record I am in no way underselling the strength of the soviet military.
they had good stuff, they were well trained for the most part, and well.. simply speaking a damn fine foe.
that said.. they had windows to try.. they never did. we had windows.. we never did.
why? no one likes to glow in the dark
Poles wanted to be free, Czechs and Hungarians wanted to be free.. im betting that it stalls, hence why those big numbers were never tried out in reality.