Some 300 tanks were converted over between '41 and '44. Thats roughly 3 times the number of wirbs produced (depending on the number you go by), and exclution due to production numbers isn't a relevant argument.
It used the Zis-4M (a Zis-2), able to penertate 140mm of steel at 500m. Probably will be perked, if not, we'll probably see another major change in tank usage, just as we did with the introduction of the M4A3(76).
Same armor, speed, and turret traverse, just had a 57mm Zis-4 in place of the 76mm F-34.
Well, what do you think? Would be very easy to add visually (just a longer, skinnier barrel).