The longer one makes me think of the T34/85 later models as compared to the T34/76 early models.
I think for practical and performance reasons, a medium tank of the era wouldn't mount a 90mm without a decent size muzzle brake, which I didn't see in either drawing. The Sherman Firefly/105mm had a fairly significant muzzle brake, if memory serves. And the Russians only mounted a 100mm gun as a fixed forward weapon in their SU100 Assault Gun to counter the Jagdpanzer.
Not discounting the merits of putting a 90mm gun on a tank of WWII era - it could do a lot of damage against tanks built to perform against 75 or 76mm projectiles. However, remember the Germans deployed 88mm guns not unlike a 90mm in only the heaviest tanks (Tiger I, etc.).
I'd suggest looking at a more modern 90mm like the Cockerill, or the ones that were mounted on the M48 tank to get ideas if you want something light enough to fit to a medium tank. Otherwise, you're probably building a heavy for it.