At 390 fly wheel horse - 15% drivetrain lose = 331.5. Ford underrates their stuff since the 99 Cobra screw up. Most stock ones I've heard about make much more than that stock(340-350) on a Mustang dyno. But dynos are different, I hear Mustang dynos have a more accurate measure because they add resistance to the rollers. A dyno jet will read 15-20% more than a Mustang dyno. So if your car was done on a dyno jet then it would really be about 408 on a Mustang dyno. The real question about how much power you have is what does it run in the 1/4 mile, and what is the trap speed.