There must be a lot of Soviet specific stowage.
Besides various ammunition boxes, rocket boxes would be good for stowing on the rear mudguards of the Studebaker (there are photographs of them in the Tankograd Studebaker book).
Bedframe spaced armour might be useful. Both sprues could be used to equip one tank.
Replacement fuel drums (based on the improved ones from the tank destroyer kit).
Skis?
Snow shoes?