If you have a stock pushrod hanging around, just use it for the test. It doesn't need to be hardened pushrod for the test. Mark the top of the valve with a magic marker., and assemble one rocker and preload it. Turn the engine over so that it goes through one cycle, and then pull the rocker off. The top of the valve should be distinctly rubbed off in the center. Odds are the stock length are going to do it. I've done a good number of Edelbrock heads in my day, and they have all come up stock pushrod length.
Kurt