Stamina,
Once you have the valve covers off your going to see the good or bad of it. I expect that you, as I was, be shocked at the mess.
Pull the rockers arms, disassemble them (as Bobby suggested, keep track of where every part cam from; in case they are still serviceable, they need to go back exactly where they came from).
A prudent and inexpensive move would be to purchase a replacement set of rockers and push rods before it's taken apart. If everything is reusable, just return the unused new parts.
Also, valve lash and hydraulic lifter preload are very nearly the opposite procedures. Ensure you understand the difference prior to attempting to perform a hydraulic lifter preload adjustment.