Hey Jim, I must be missing something here. Is this a Chevy motor you are working on?
I remember the push in studs being a problem as they can work out of the head. We used a tool to check the stud height from the deck when we had a lifter ticking. More often than not we drove the stud to the proper height and the lifter issue went away. (that is if the cam wasn't going flat) Different with the screw in studs as that never happened with them, they just broke.
(I still have a tool I made to remove the push in studs when they broke off clean with the head.) If the stud became loose I remember they had an oversized stud you reamed the hole and drove in the new stud.
I don't ever remember seeing an adjustable stud in a Pontiac I worked on, always ran them down to 20ftlbs and called it good.
Chevys were fully adjustable in that day as I hated doing those...oily mess especially with the big oil pump, squirting oil over the fenders :-)
Not my engine, its Hotsticker1
as this is his post.
But to answer your question, and it sounds like you had your share of work on them, the early/low performance Pontiac heads used the pressed-in studs. These can pull up if you start going bigger cam/stronger valve springs or probably revving them way more than they should go on a regular basis.
In 1967, the GTO and high performance heads used screw-in bottle neck rocker studs (7/16" at the base tapering down to a 3/8" threaded top - thus "bottle neck").
Both the press-in and screw-in studs used the torque-down method rocker arm nut where you simply cinched it down to 20-25 ft lbs of torque and were done with it. However, to increase the useable revs available in the engine, the trick was to adjust the lifters to "zero lash" using the poly locks. The "Bobcat" performance package that was available for the Pontiac included a rocker arm nut that was more like a nylock type locking nut where it would not back off like the stock nut would if it were not torqued down.
(I have a set of clips made specifically for snapping right onto the top of the rocker arm covering the oil spurt holes and minimizing all the oily mess. Bought them 35 years ago and still use them. LOL)
By backing off the rocker arm nuts while the engine was running would cause the lifter to start clicking. At that point, you would turn the nut back down until the clicking stopped, then turn the nylon lock not (or the poly lock's) 1/4 turn more. This put the plunger in the lifter at a higher point within its body and prevented lifter pump - kind of like turning the hydraulic lifter into a bit of a solid lifter. This gained more useable RPM's out of the engine so you could spin it more like 6,000 RPM's.
As you pointed out, the bottle neck screw-in studs could break off at the bottle neck where it tapered down. So the fix, and what most do in their rebuild and when going with a bigger cam, is to use an ARP brand Big Block screw-in rocker arm stud which is 7/16" from top to bottom and fits right in. Then you have to use the poly lock's to lock down the rocker arms.
If the heads/block have been milled and the valve train has not been adjusted to compensate, the hydraulic lifters/valves could be adjusted just a tad too tight using the factory nuts which could hold the valve open just enough to create an engine vacuum problem. Same goes with the poly locks when using them to adjust the lifters/valves. Just something to watch for and double check if having low or erratic vacuum readings.