If PA takes place, say, six weeks before the Expo, Tony showing up at Charlotte's and giving her the car and wanting her to head up this new charitable division of SI makes a whole lot of sense considering what's happening to Tony health wise. So the sequel picks up that same day, and the events I've already partly written, like Tony discovering Charlotte is Immortal, take place just before the Expo, then just before Monte Carlo and the race. The new stuff, like the race restaurant and birthday party scenes, I just fold into the framework. I think it could work with a little tinkering. Of course, this will be a much larger undertaking that just a story set during the birthday party, but on the other hand, it finally gets the sequel written!