The Western Allies don't need to declare war on the Soviet Union, just not accept them as allies.
This would actually suit American ambitions nicely as well. By not forming an alliance, The Soviets are not bound to respect British spheres of influence in the middle east, the Soviets would quite happily continue espionage and insurgency efforts in the middle east, Greece etc which would in time weaken Britain for the post war period.
The Soviets would finally overwhelm the Nazis. By sheer weight of numbers if nothing else, but, American food and Trucks aside, the Soviets could and did produce pretty much everything else themselves. It could potentially drag the war in Europe onwards, certainly on the Eastern Front, but it does leave the Western Allies free to persue a seperate peace.
So, post war in the event of a seperate peace?
The Soviet Union would be much weaker physically, but much stronger ideologically so after a period of recovery, probably about 20 years, the COmmunist bloc could last through to the 21st century. Soviet Territory: Soviet Poland and Soviet Finland are a very real possibility. If there is no one else to guarentee its existance, Germany could well disappear into seperate Soviet States with potentially France, with British and US backing, annexing as far as the Rheinland before the Soviets got there.
With no one looking over their shoulder, I think the Soviets would roll straight into and over Finland. Sweden would be watching very very closely and I suspect that the Western Allies would have to guarentee Sweden's "Neutrality" to prevent the Soviets trying to make a claim on parts of Sweden they fancy, however Finland would really be the last possible extension until VE +20yrs, (roughly).
Far East: Land War rolls back at a slightly faster pace, but not much. Island Hopping campaign again moves slightly faster with the home Islands under siege by May 1945. I'm not sure the Japanese could physically survive 3 more months of siege, but if they do, Fat man and little boy pay a visit as per OTL. Differences: No split Korea and no split Vietnam. No Korean war although I think the Vietnam war will be close to the OLT issues, but across all of FIC. Soviets not really able to give much support to the Chi-Coms, whilst west offer more support to Chi-Nats. China will explode into civil war eventually with a Chi-Com win, but I think you're looking at VJ +10 - +15 years before hand.
Of course, this is just my opinion based on gut instinct and with no real research, but that could make for an interesting timeline.