Ford decides which models they will offer x-plan prices on, and which they don't. In other words, there are some models for which there aren't any x-plan deals, and the dealership has no say in it. So, there is no guarantee at this time, that any of the '05 Mustangs will be included.
It is interesting, I can't find a rule about dealers being able to selectively pick models in the rules on fordpartner.com. It just says that it is up to the dealership to sell under the x-plan or not. I mean, there doesn't seem to be rule either way.