HP/Crank HP = The amount of power the engine produces at the flywheel as measured by an engine dyno.
RWHP (Rear Wheel Horse Power) = The amount of horsepower measured at the rear wheels by a chassis dyno. This is what really counts, where the rubber meets the road so to speak. This number can be 10-20% less than the crank HP because some of the horsepower gets used up by the drive train (transmission/rear end).
The '03/'04 Cobra's are under-rated at 390HP (crank) HP from the factory which equates to about 330-380 RWHP depending on the make/type of chassis dyno used for measurement.