Bring the engine to TDC of #1.

Pull off the drivers valve cover and remove the rockershaft assembly. Strip off all the rockers except for #1 int and ex.

Reinstall shaft and finger tighten the adjacent hold down bolts until any pushrod lash is gone.

Lay a straightedge on the valve cover rail and lightly scribe a line on the 2 pushrods.

Tighten the bolts down completely and rescribe.

Remove the rocker shaft and measure the distance, thats your actual lifter preload.

That will tell you if your pushrods are the right length.