It's also questionable if they'd manage to catch the Soviets with their pants down like they did IOTL. Think about this series of events from Stalin's perspective:
-Britain, Poland and France allowed for Hitler to take over Czechoslovakia.
-Britain and France turned down Soviet attempts at creating an anti-Hitler pact (unless Litvinov doesn't even try in this timeline)
-German and Polish right wing governments are settling their dispute peacefully, both are very hostile to the USSR.
-Just maybe, I should bolster the numbers and readyness of the troops in Belarus and the Ukraine.
Interestingly with Poles in even Ukrainians may be more inclined to fight for Soviets from start.
On other side, let say it happens in 1940. Against France Germans fielded 141 division, add at least 39 Poland's divisions and some 16 brigades (fully mobilized). Plus bit more modernized Polish air force. Still Polish Air Force would be very possible doing pretty well against Soviets in 1940. OTL Romanian Polish fighters show down over 30 Soviet bombers in 1941.
OTL Finland and Romania fielded against USSR in 1941 14 and 13 divisions.
I guess with no Ribbentrop-Molotov Pact there will be no Winter war and no occupation of Besarabia. Much less reasons for Finaland and Romania to join. Now on Romania Germans can put much more pressure then on Finland. With Norway unoccupied, no Winter war and Soviet Union attacked actually its Germans who are sean as bigger danger in Finland. I would scratched Finland out.
If Baltic countries are attacked by German/ Polish alliance they are on one side quickly defeated on other they are pushed to Soviet camp without Soviet pressure of 1939/40.
Romania. How you get Romania in if Bessarabia is still Romanian? Threaten them with OTL 2nd Vienna Accord? It will push them more to Soviet camp or at least make them neutral. Especially if Soviets said they are fully respecting Romania pre 1939 borders. Romanians were trying to balance their relations between France and Germany OTL till fall of France. With France intact i do not think you can make them to commit to war against Soviet Union. Hungarians were hasitant as it went OTL 1941. They were trying to balance between Italy, Germany and western allies as much as they could too. They didn't even confirm their commitments against Czechoslovakia during Munich crisis. Of course they always used opportunity to regain territories of old Hungarian crown.
Will Italy send some expeditionary Forces? Possible. They can even supply more Fiat trucks to Poland.
So just Poland and Germany, let say with neutrality of Baltic states broken by Berlin-Warsaw Axis front goes from Baltic to Kamenets Podolsky However before Baltic states are gone, Soviets have some time prepare defense there. So intially forn goes somewhere along Polish Soviet Border from along Daugava river to Kamentets Podolsky.
Soviet army is unaware f problems it has due to no 1939 Polish campaign and no Winter war. On other side. Germans do not have experience with Polish, Norway, low countries and France campaigns. Same goes for Poland. Soviets at least do have some experience from Chalchyn Gol. How much good it will do to them is different story though. On other side Stalin's line is not dismantled.
Without Polish aggression against Poland and Finland France and British do not have reason to support this Axis. Actually due to Soviet France alliance from 1935 France would be actually on Soviet side. It would do to Soviets probably same good it did to Czechoslovakia, on other side they could expect at least some aid. Plus strong leftist parties in France may push France to aid Soviets in some way - technically, financially, diplomatically if not directly engaging at war. Plus keep Romania and Turkey on right side of border by their political pressure.
Without Germany able to move at least some assets down the Danube to Black Sea and without Romanian navy, Black sea is more or less Soviet lake - unless Germans managed to Swing from Belaruss, northern Ukraine towards Odessa and Crimea. Which would make their logisitcs, withou Romanian and Hungarian railways... pain in the ass. Of course Baltic is German lake with some nuisance rides of Soviets subs and Air Force.