What will cause slight miss idling on a 65 170?

Adjusting the valves running does not splash oil out at idle on a slant 6, and does not beat up feeler gages. And unless someone adjsuted the valves worng, they are not likely to be tight; they will wear towards loose.

You don't need to be exactly on the 120 degree marks to do it that way, just close. It's worthwhile to do this with an unknown engine; mine must never have had been adjusted when I got it!