According to the 1999 Ford Mustang owners manual :
The GT model came from the factory with a :
( 281 cubic inch / 4.6 liter Single Over Head Cam V8 engine )
* it is rated at 260 horsepower at 5,250 RPM and 302 lb-ft of torque at
4,000 RPM
The 2001 Ford Mustang has a V6 engine.
The 2010 Ford Mustang has a V6 engine.
The 2008 Ford Mustang has a V6 engine.
The 2002 Ford Mustang has a V6 engine.
The 2013 Ford Mustang has a V6 engine.
The 2004 Ford Mustang has a V6 engine.
The 2009 Ford Mustang has a V6 engine.
The 2007 Ford Mustang has a V6 engine.
The 2014 Ford Mustang has a V6 engine.
The 2011 Ford Mustang has a V6 engine.
The 2003 Ford Mustang has a V6 engine.
The 2005 Ford Mustang has a V6 engine.