You must have the lifters on the base circle, ie, valve fully closed to adjust them.
Two ways:
a-rotate the engine and set them in order of the firing order, one cylinder at a time.
b-You can index the motor to three positions and it will get all 8 cyls on the base circle. The positions are in most books on sbfs.
I prefer just going through the firing order one at a time, as its easier not to miss anything.