75s should still be over 250m in height, so 120 Harbour would be far from short. But yes, it would need to be at least 85s to have a reasonable chance of exceeding 300m (unless Tridel goes all Dubai and decides on 4m+ floor heights, or else adds a massive spire).
Even so, when you add the (very likely) several 200m+ towers that will eventually go up on 60 Harbour, plus the likely 200m+ tower on 45 Bay, this whole South Core district will have a VERY impressive skyline, rivaling cities like Calgary just by itself. (Certainly not as large in terms of numbers of buildings, but in terms of height and density, it must be close.)