whatever year mustang you get; 4.6 DOHC or SOHC, stick with it. You don’t need to swap for a 5.4 to go fast, Theres to much involved and might become a head ache( I’m not saying it can’t be done). I’m sure the DOHC or SOHC will be fast enough for you. There are many fast cars that are using 4.6’s