It can be done and was done already by a Bullitt owner, its no where near worth it to do. You will easily spend a minimum of 10,000 dollars just to do a new 5.0 engine and transmission swap. Just to fabricate the headers was 2,000 dollars and that was just the headers.
EDIT: I should say right now its not worth it to do. Maybe in a few years when the aftermarket catches on, they might make a affordable kit to do the whole swap