as long as you adjust a rocker nut when the lifter is all the way donw in the bore of the block and the foot of the lifter is on the heel of the cam lobe then you can adjust it
i like to adjust them before the intake manifold goes on
otherwise follow the repair manual for what ones to adjust with TDC #1 and TDC #6 firing positions
richard got you covered with what the books tell you to do
tighten down just until rocker slop is gone barely and then tighten almost another full turn is what i like to do
better to be a hair looser than tighter. they are hydraulic anyways and will take up the slop within reasonable dimensions if they are in good condition and not worn collapsed on the feet of them.
if any of them are then pull the cam and put a new one in with new lifters. these engines are rally bad about wiping cam lobes off
common problem on the old engines with alot of miles on them..
if you adjust them all and end up with a pop up the intake or backfire or bad running and its not due to the carb or ignition system then you may have a wiped lobe or two, and/or stretched timing chain needing replaced, also a very common issue with these old engines.