If you install milled heads or a smaller base circle cam (often required for more valve lift than stock) or make any changes to the top end of the engine, you'll need to correct the valvetrain geometry. The best way to do this is with different length aftermarket pushrods. This gets the geometry right but also adds strength since the factory pushrods are too lightweight for high-rpm valvespring pressures. So swapping them for performance pushrods is required anyway.
Gen III GM Small Block Tech