In Les Andrews Vol 1 book, checking the front spring for sag, one is to measure the distance between the middle of the bumper and floor. On page 1-275 these values are listed:
28-29 = 18 11/16 +- 1/2"
30-31 = 18 1/2 +-1/2"
On page 1 -279 the values are swapped between the year groups...