I have always found that trying to install the rocker arm assembly similar to trying to hold a wiggling, squiggling baby.
Set up the assembly on the bench and by the time you have picked it up and moved it over the head several rockers or mounting stands have rotated and you can’t get on the head.
Several years ago I hit upon a solution:
Set up the rocker assembly on the bench and place a ruler on top of the rockers. Hold the ruler firmly on the assembly and the whole thing behaves.
Picture attached. Although this is a race engine, I have used the same procedure on stock heads.
Try it, maybe you’ll like it.