I dropped an e-mail to a chap who imports GT500s - someone I've spoken with in the past (a realist, not a silly-mark-up kinda chap) and asked him what the chances were, at reasonable price.
The best route would be to call Ford dealers in the US but I've done that once and it takes days and is a real nightmare.
We'll see... it's a nice to have, not a must have but I have a few American cars which are on my list to own and I have to say a modern (and a 60's) Mustang are on the list.