I am not sure about the exact frames, but I am familiar with the timing, I can consistently hit Shang while he is unable to block if I am looking for it. The easiest way is a set situation, like a restand, or off the hitstun, when you know he is going into recovery and can’t delay the morph back by staying in a block animation.
Like I mentioned before throw isn’t always a good idea. For example Geras’s throw leaves him close and the animation is so long that normal Soul Steal is guaranteed to expire. There isn’t enough hit advantage for you to safely morphback after, and Geras can punish you with his wake up or at least start his pressure/throw game.
And yes I am the one suggesting the safe morphback or early morphback at the cost of a defensive bar.