The proper way is to buy an adjustable set of pushrods used for determining pushrod lenght. They come in a set. You pick out the correct lenght and it has a range of adjustments with two jam nuts. It’s made of aluminum so don’t turn the motor over with springs on the heads. It will bend them. Make sure you have the correct preload on the lifter with the proper amount of threads showing on top and bottom of the rocker. Then you remove this pushrods and measure it with a dial caliper. Manton or Smith Bros can build you the correct lenght pushrod.