I should have mentioned that there are two basic types of rockers. I have adjusted them on the 2.8 L version of this engine. With that engine, the bolt that held the rocker down was also its bearing surface. There was nothing under the rocker but the stud. The rocker was suspended between pushrod and the valve and the spring inside the lifter provided the tension to hold the lifter in place when it was not up on a lobe. This was most common on Chevrolets.
The old Pontiacs had a bushing that went through the rocker and rested on a seat on the stud. This one was just torqued down. The torque was the bushing against the seat and this type was not adjustable. If the lifters started ticking, they had to be replaced, no adjustment.