On the frame drivers side under the hood there is a number that "should" match the last numbers on your VIN tag located on the door jamb or top door jamb or rear door jamb depending on the year. Then go here for more info:
http://67-72chevytrucks.com/vboard/s...d.php?t=463671
or here:
http://67-72chevytrucks.com/vboard/s...=316902&page=4