Hydraulic Lifters?
 

My son has a 1978 Sierra. We have conflicting info in manuals. One says to torque down the rocker arm pivot nuts to 18 -25 ft. lbs. Another says these are used to adjust valve lash. This confusion is probably because one info is for hydralic lifters and the other for non-hydraulics. Question: How can we tell the difference in the lifters? Which do we have?

Well to start you are almost guaranteed to have hydraulic lifters unless someone has modified the engine and added solid lifters but from the factory it would be hydraulic. i am going to assume it is a small block like a 350 or whatever and here is what you do. To adjust the valves grab the pushrod and move it up and down while turning it and tighten the rocker arm nut down. When zero lash occurs that is when the pushrod has no up and down movement and no longer turns you should give the wrench 1 full turn after zero lash to set lifter preload. Of course do this with the cylinder a top dead center when the valves are closed. Your manuals sound very odd and I have never heard of a torque spec for rocker arms but what I told you is the way I was taught and believe is the proper way to do it.