It's next to impossible to tell if the engine is the original one for a classic mustang..
Mustangs built before 1968 do not have the Vehicle Identification Number (VIN) stamped into the engine block. The one exception is that all "K Code" Mustangs with the 298 High Performance (Hi-Po) package had the Vehicle ID stamped into the block as a theft deterant..
However, things like later model date coded parts on an early mustang are a good sign of major repairs or of a possibly changed engine. If one sees "C7" coded parts (1967) on a 1964 1/2 vehicle it's not safe to assume the engine is "original"
You can also check some of the cast items on the car for their casting date. You can compare the casting date to the build date of the car (on the warrentte plate). If your engine block or heads are the correct year, but were cast after the build date of the car you are fairly sure it is not original.
On the driver's side of the engine block you should see either 318 or 360 stamped.
I'll sell you my all-original 3.3 liter engine out of a 1966 mustang for $750 If it's original it will be worth more. A larger motor will be in the thousands
Will a 1998 3.8 mustang engine fit in a 2000 Mustang 3.8 engine?
The 2006 Ford Mustang has a V6 engine.
The Ford Mustang GT has a V8 engine, while the Ford Mustang has a V6 engine.
The 2005 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 2009 Ford Mustang has a V6 engine.
The 2012 Ford Mustang has a V6 engine.
The 2004 Ford Mustang has a V6 engine.