A V8 Mustang would have a V8 engine code as the 5th digit in the VIN, such as "6T02C", indicating a 289 2V engine. The 4V would be an A, and the 289HP would be a K.
If you go to Classic Mustang Dataplate Decoder and put your vin # in the appropriate block along with info from the door tag it will tell the body style,color, eng, trans,rear gear ratio,and int color.