There was a show on Power Block on SPIKE yesterday that had this trick tool to determine the proper length push rods when adding a cam. You mark the top of the valve, you slide the tool down in the push rod hole (which the tool is adjustable) and then you install the rocker arm you are using. You rotate the engine, then you take the rocker off. If the mark in the paint on the valve is in the center, you have the right push rod length. If it is to high, you need a longer push rod. If it is too far down, you need a shorter pushrod.