A light tank and a medium tank with the same roles and a 30 fuel 120(?) gap... Currently, everyone would go make t34's straight away, because as you said Germans already have the PIV out. The T70 was mainly made to play in the mid game before late game medium tanks comes out to harass the light armor before real German AT rolls out. This situation only happens within soviet t3, so it doesnt really belong in t3 anymore if you can get a tougher alternative at the same time for a little more. I hope the armor patch fixes all soviet tanks so they are in viable locations in t3 and t4, and useful in the game. (Currently Su-76m is on the list of rarity, and t-70 just landed on that list as well. A switcheroo in tiers with those two and changes to the su-76 seem like one way to fix their purposes.)
Imo soviet T3 needs to be a lot cheaper fuel wise, that is both the building and the units. If we fix the T-34 at where it is right now, the building itself shouldn't cost much more than T2. If additionally fuel cost is lowered by 20 fuel, it could be used vs. german T2. The MG truck could see some use on par with the OH HT.