How do I dynamically reposition part of a sentence based on the width of a word in the middle of the sentence that will change? I am looking to build an animation where in the middle of a sentence, a word gets deleted by character, then a new word is retyped on-screen and the back half of the sentence repositions as each letter is typed on. I tried to do some research and have a feeling this animation may need to utilize sourceRectAtTime or Delta expressions along with the type-on character text animator. What are your thoughts on accomplishing this?
I think you could achieve this just by key-framing the Source Text control in your text layer.
Unless you have hundreds of them and need to do it programatically, I would just keyframe the changes of text, and the repositioning would happen automatically. 
If I have misunderstood what you mean, please add some screen shots to explain what you need.