The Barrel is certainly longer.
Basically, if you could fit the Zis-2 you could fit the 76mm Zis-3, however the supply of replacement tank commanders might get a bit difficult.
Tank guns generally had much shorter recoil than towed guns even if they used the same ammo. Different hydraulic cylinders, buffers, recuperaters. A towed gun could have the breech block recoiling right through the tank commander if you just "stuck' it in the front of the turret. The shorter recoil distance increases the peak recoil load.
Granted the Germans could design and build a different recoil system but you are getting into a gun of such power that it is almost a 75mm/L43 in size and weight.