When you called J/S admin, did they give you a confirmed reservation? As I understand FedEx's offline booking process, every offline pilot is assigned a seat in the cockpit and it is Capt's discretion to have them sit in the back.

So, if the j/s folks told you that you have a ticket to ride, should be good to go. If you have a flight number or city+local date, sure there somebody will take a look at the plane type for ya.