Few automobiles have had the sort of longstanding street cred given to Ford’s Mustang GT. But the deference it has enjoyed for much of its four-plus decades has been called into question recently now that recently introduced muscle-machines from Chevy and Dodge have come along with larger, high-tech V-8s. With its comparatively small 4.6-liter V-8 under the hood, the Mustang has had a lot to