The regular GT Mustangs are not supercharged. Depending on the year, if it was stock, it was probably around 300-325 hp, less if it's an older model. The new 5.0 liter GTs are at 400 hp, also not supercharged. Only the GT500's are supercharged, 500 hp or more. Unless your 440 is built pretty strong, you'd have a tough time beating one of those.
Still - nice kill!