Perhaps in a "not-invented-here" vein similar to the German attitude that produced the Panther, the Allied designers would insist on "improving" the T-34. The thing you SHOULDN'T mess with is it's mobility, so the wide tracks, Christie suspension and 500bhp engine would stay. For an allied version to be available in time to be useful, it would have to be supplied pretty early, so the exemplar vehicle would likely be a T-34/76, whose 2-man turret wouldn't be popular. You might therefore see the Western T-34 having straighter hull sides and a bigger 3-man turret along the lines of the Sherman.