Don't get me wrong, I'd buy a Mustang with a larger displacement engine in a heartbeat, but the engine would have to produce more power than what the Mustang currently has. If Ford made a Mustang with a 7.3 making about 700 NA HP I would definitely want one.
From a Road & Track article:
"And if you're wondering if it'll fit in a Mustang, Beltramo told us it's actually shorter in height and narrower than a Coyote V-8, but quite a bit longer thanks to much increased bore centers. It's possible then, but good luck doing so without having to cut into the firewall. And even if you could wedge this motor into a Mustang, it wouldn't be very good, because of its heavy cast iron block and low-revving nature. Stick with the Coyote, kids."
Would still be an interesting swap though, I'm sure somebody will be crazy enough to race car a Mustang with one of them.