That O' Reilly head is probably a remanufactured head, not a new one. The year of the truck determines whether it has manual or hydraulic lifters. Manual adjusters were used up to and including 1984 motors, 1985 and up are hydraulic lash adjusters. You can get aftermarket heads either way - just have to specify when ordering. It would help if you tell them it is a California truck to ensure the easiest install.