Have you ever heard of valve hushers? I'm not sure if that is the correct technical name but I have seen it in this board. They are rubber O-rings on the top of the valve stems under the cam followers or tappets. They prevent the clicking noise normally heard in non-hydraulic valve systems. I suspect all the clearance is taken up by these.

These rubber hushers must be compressed by pushing down on the follower before inserting the feeler gauge. You can feel a definite stop when they are fully compressed and the follower hits the valve stem. The valve spring is much stiffer than the hushers so there is no danger of pushing the follower down too far.

If you cannot get conformation of this here try turbobricks.

If the ruff idle were a valve problem a compression check should detect it. Rough idle may be a lean mixture condition causing intermittent missing. You should be using original copper core type plugs. Check for good spark at each plug and use a vacuum gauge to adjust the mixture. After that check for vacuum leaks.

I assume an ’80 still has the idle mixture thumb screw under the intake manifold. If not and you have an O2 sensor you can use its’ electrical signal to adjust the mixture in the air meter.