A big can of valve lash may fix it, but it may need more. Run the motor warm with the rocker covers off to see if you have oil coming out the little pipe on the right end of each side. If not, dont wast time adjusting things, you need to go deeper. If you have a good supply of oil, and it has been making noise a long time, chances are there is a groove worn in each rocker arm. That will make it nearly impossible to get an accurate adjustment. The book calls for .019. You can adjust them to .016 to try and take up some of the play. Any less may not be safe.