I'm sure it's just speculation, but at the bottom of the article they mention the Trinity fitting with some modification.
Ford Mustang (2015) CAR inteviews the chief engineer | Automotive & Motoring News | Car Magazine Online
"Will there be a Shelby version of the latest Mustang? Can the 662bhp V8 from the outgoing GT500 fit?
The 5.8-litre supercharged engine? We’ve not excluded it from fitting. Modifications will have to be made, but will it go under the hood? Yes, it will…"
Ford Mustang (2015) CAR inteviews the chief engineer | Automotive & Motoring News | Car Magazine Online
"Will there be a Shelby version of the latest Mustang? Can the 662bhp V8 from the outgoing GT500 fit?
The 5.8-litre supercharged engine? We’ve not excluded it from fitting. Modifications will have to be made, but will it go under the hood? Yes, it will…"