you have to measure them at a specific point in the cam cycle to determine if they are failing. All cams have an overlap that will compress both intake and exhaust lifter at the same time. At ignition timing, both valves are closed for that cylinder. Good time to check lash (or lack of) on a hydro lifter. All of them have a small amount of preload on them so you should not be able to rattle the pushrod on any of them. Who knows hat has happened ion that motor in the past 48 years! No solid lifter will use a stamped non adjustable rocker. Wear pattern on a lifter may not be visible but it can be seen if you take the lifter face and put it against another lifter face. It should be convex, and be able to rock on the other lifter. If it is completely flat or even concave (dished) then it is worn out. I've seen lots of concave solid lifters in slants and they still run fine. Most have adjustable rockers than mask this lifter condition.
