If you’ve severely decked the block or heads, run a different-thickness head gasket, added longer valves, changed the rocker ratio, swapped in different-length lifters, and/or put in a reduced base circle or reground cam, stock-length factory pushrods likely won’t achieve correct valvetrain geometry. The engine won’t get full camshaft lift, plus the possibility of pushrod, rocker-arm, valve-stem, and valveguide wear or damage increases significantly.
In a perfect world, a rocker arm would always push against the valve stem’s exact center. But in the real world, the arm travels in an arc as it moves up and down, so you have to allow for this when setting up the valvetrain. To check the geometry, properly lash the valves, then rotate the engine through a complete valve opening-and-closing cycle. (On a hydraulic…