Since you have a 351 Mach 1 (body style 63), according to the MPC it lists: C9ZA-5556-F, or D0ZA-5556-G, or D0ZA-5556-H
Most likely this is due to engineering changes that occurred during the model year. As is often the case, your build date and assembly plant will play a role in determining the best choice. Will need input from owners of similar cars.
With no other information, and a car that late in the model year, I would lean towards either D0ZA-5556-G or D0ZA-5556-H