I loosen the locking nut and turn the screw alittle on the rocker arm.
Post the engine serial number on the pad/ledge on the block that is just to the rear of where the distributor goes into the block. This will assure you/us that you have a truck engine (all of which came from the factory with mechanical lifters). A 58-63 truck engine could have been later fitted with hydraulic lifters.
In case the posts above are not clear, your adjustment technique is not the recommended technique for mechanical lifters
HrL describes the accepted/recommended technique. A few techniques are described in the Engine secton of the Tech Tips on this webite (look for the Tech Tips link at the top of this page).
Here is a description of an adjustment technique. 
