Yes the Mach 1 gets the "old" Cobra Motor.
The GT's will likely stay the same until 2005 when the redesign happens and then I've heard there going to take the 4.6 to 5.0L's. That would be a great marketing strategy, "...the New 2005 Mustang with the 5.0L V8!" People may not realize it's still just a 4.6, just stroked or bored out some.
Ford might be smarter than we give credit....Naw it's just the guys on team Mustang
