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Well I figure that maybe not everybody knows the exact years and what the body styles look like for the various years, so I will post some pics of the various years so everybody can give an accurate description of their favorite mustang year.
This is the 1rst Generation Mustang, this picture specifically is a 1966 Mustang Coupe.  The Horsepower and Torque varies depending on what code you have, here is a complete listing.

Code                     HP                               TRQ
C                 200 @ 4400 RPM             282 @ 2200
A                 225 @ 4800 RPM             305 @ 3200
K                 271 @ 6000 RPM             312 @ 3400
The pictures to the right here are a 1976, and a 1977 respectively. I personally think this is the worst year for the Mustang,  the 2nd Generation, dubbed by some "The Pinto Years" heh, but here are the specs.
Mustang II
This year the mustang came in 3 different engines.  The 2.3L 4 banger, (4 cylinder), the 2.8L V6, and the 302 (5.0L) V8
To the left we have the Mustang Fox Body GT.  This ran from the early 80's to 93.  A couple of changes were viewed through these years.  In 1987 the front end was shaved off and given a flatter nose.  In 1986 the introduction to the 302, 5.0L EFI V8 was introduced.  1986 was the first year for the mustang to have the Electronic Fuel Injected 302.

The later Fox Body's made about 225 HP, where as the earlier ones were around 205.  However the 5.0 Pushrod motor is very easily modded, and can make a substantially more power over stock.
The lower two pictures are linked to their prospective owners sites, click the pictures to learn more about their cars.
The 94-98 Mustangs, led to many many changes.  Lets start with Cosmetic changes, this is easily seen.  The no longer boxy Fox Body is seen.  Instead a much more curvaceous body has emerged.  This is the start of the SN95 Chassis.  But body styling isn't the only change,,,, the 5.0 was kept for 2 more years.  The 94 and 95 GT's still kept the 5.0L 302 Engine.  These years also had the Horizontal Lights on the back.  In 1996, was the start of the 4.6L or the 281 Engine.  This also introduced the vertiacal 3 chambered tail lights.  The 4.6L is often criticicized for their "crappy heads" b/c they didnt flow as well, as the newer 99's do.  The 94-95's have a very wide Modification field open to them.  Making 225 HP stock, but easily modded.  The 96-98's make about 215 HP,,,,the years vary slightly ranging about 5hp.  This is also the very end of the damn 2.3L 4 Banger, we can finally say sayonara (forever, hopefully) to that dreaded 88HP BEAST..hahaha.
The most recent Generation, the 99-current,,,,,,,this had some subtle body changes, still keeping a similar look, but adding a slight "retro" feel to it.  The redisigned engine now puts out 260HP where the V6 puts out 190, quite an improvement over the 215 for the GT, and the 150 for the V6.