If you find a 04 Mustang body and it is a 4.6 body car already then that engine from 04 is better. To put that engine you have from the thunderbird down into the car it is alot of work for minimal gain over an 04 v6 car. Plus the amount of items you need to make it work. I am currently working on a 99 v6 5 speed car swapping it to a 4.6 5 speed and have almost everything I need to swap it. Alot of work there but it is a project car for me and my son. You can do what you want, I just dont see the purpose in an older 4.6 that didnt make that much power going into a new edge mustang.