From 1981 - 1987 Chevy Trucks used a 17 digit VIN#.
If your 1973 - 1987 body style has a VIN# located on the dashboard, with 17 digits, then it is between 1981 and 1987 model.
If your truck has the VIN# on the dash, but it is not 17 digits, then it is a 1979 or 1981 model.
From 1973 - 1978 Chevy Trucks had the VIN# on a plate, mounted to the driver's door jamb.
Your VIN# can also be found on the factory options decal, located in the glove box. The decal lists all the RPO's (Regular Production Options) your truck came from the factory equipped with. If the 4th digit of your VIN # is a 4, then your truck is a "Heavy Half" ton, also known as a "Big 10".