Just to be clear, it sounds like you want to have a SS with the task's predecessor and a FF with its Successor, correct? If so, that would mean for the task to really finish with its successor, its duration would need to change if task has already started with the SS and the successor'd finish date was extended. MS Project will not automatically change a task's duration based on the connections. If that task has started, the end date will not move regardless if the FF successor slides to the right. You would need to either a SS pred or a FF successor, but not both. I have engineers ask me to do this all the time and I have to break the bad news to them. Hope this helped.