What will cause slight miss idling on a 65 170?

Take the choke spring assembly out of the choke well on the intake. Make sure it is clean. You will see a small nut on the sit and a disc with markings L and R (lean and rich). You can loosen the nut and move the disc towards R to make the choek come on harder or to L to have less choke.

A slight miss once on a while is not something to be overly concerned with, IMO, especially with old points and such. It could be a tiny vaccum leak, a valve sticking a bit, loose valve guides (this is actually pretty likely with high mileage and age), or one erratic spark once in a while. Get it tagged and drive it to see if gets any worse.