Hello! Two styles of rocker studs were used on the 4.3L engine. Since teh "screw in" style with postive stop shoulders is most common, thats probably the reason why you got those instructions. However, some heads still used the older style "press in" studs, which require the manual adjustment.
Back off adjusting nut until lash is felt at pushrod, then tighten nut until all lash is removed. This can be determined by rotating pushrod while turning adjusting nut. When all lash is removed, tighten adjusting nut 1 and 3/4 turns to center lifter plunger.
Crank engine one full revolution until mark on torsional damper and "0" mark on timing tab are again in alignment. This is the No. 4 cylinder firing position. With engine in this position, adjust the following valves:
I listed all the cylinders to be adjusted, because I don't know what head you removed. You only need to adjust the valves that are on the head you replaced.
Unfortunately, that is possible. I would remove the rocker arms, pressurize the cylinders to see if the valves are sealing and to inspect the pushrods. The lifters are usually the last item to get damaged.
If all checks out, go ahead with the adjustment procedure and see how it does.