I can't say for sure that this works for 1st Gen trucks......BUT...when I was checking for sag on my 8" Skyjacker front springs, they told me to measure "free arch". With the spring off the truck, with the spring eyes on the ground and the centering pin up in the air, measure from the main leaf to the ground. For my 79 8" spring, it was supposed to have around 9" or free arch.