On a pushrod engine like an LS, and every other American V8 that doesn’t have overhead camshafts, the length and width of them is pretty critical to insure performance and longevity. Different engine platforms have different rocker arm configurations, with some being adjustable and others not. The shaft mounted rocker assemblies in an LS engine are NOT adjustable which is not an issue with factory LS engine combinations as they all came with hydraulic roller lifters. Camshaft changes, cylinder head and head gaskets changes, rocker changes, etc can all effect how the pushrod, camshaft, and rockers fit together so when changes have been made it is smart to check their fit and measure for proper length.
There are a few reasons for making sure your pushrods are the right length, but in the most basic terms the length will dictate where the rocker arm rides on the end of the valve. If the pushrod is short the tip of the rocker that rides on the valve will be offset to one side of the tip. If the pushrod is long it will be offset the other way. What we want is a rocker arm that rides right in the middle of the valve so that it is pushing on it in the most efficient manner, without causing excess wear. Yes, there are other things that can be used to help with this geometry, like lash caps, but usually that isn’t required.
In this video, Richard Holdener goes into how to measure pushrods and why, with detailed video instructions. If you have NEVER installed rocker arms, pushrods, etc, that is okay. This video will show you everything you need to know.