Setting the lash with the engine running was the only way we knew back in the 50's and early 60's. I have The valve adjusting tool and a go,no go feeler gauge. We set the valves on the newer engines with hydraulic lifters by backing the nut on the rocker arm off until it you could hear it click then tighten up until the clicking stopped and then maybe a quarter of a turn further. I believe that is the way the manuals said to do it.
Jim
