They made the 427 bodies wider than the 289 by simply making the tops of the fenders a couple of inches wider on those seams. You can see a neat picture of the 289 buck with a flare added to it here
https://www.facebook.com/photo.php?f...type=3&theaterThe flares were also added to many 289 comp cars and standard on the 427 cars (except narrow hip). The fender flares varied a lot in size and shape as they were hand made and often to accommodate more wheel and rubber for the track. Mine measures about 70 inches over the rear flares but this and most dimension sheets says they were 68" wide.
1966 AC Cobra 427 specifications, information, data, photos 51678