318 Engine stumble

Your secondaries are too far open, and the high vacuum is sucking mixture and ari through them at idle. Close them back down, but make sure the butterflies are not touching the bores. Being adjusted to 'look like a square' is just a starting point; adjustment up or down from there is common.

In your driveway, where this is acting up, the carb will be operating just on the primaries, and is going through the progressives (or transition slots) when this happens. If it is not ignition related, then I would first check float level and then pull off the meter block and clean it out; there may be some crud in the idle wells.

Once the secondary stop screw is changed, see how far your mixture screws are opened; 1-2 turns is 'normal'.

BTW, I see you have a points distributor. If the points have closed up (due to rubbing block wear is a common cause), then it will show this symptom of low RPM stumble. Adjust the points gap to to be about .017" open at maximum opening. (Or for proper dwell angle of 31-32 degrees if you have a dwell meter.) If the points' rubbing block has not been properly lubricated and has worn down the 'pointy nubs' on the octagonal cam, then you can adjust the points gap all day long and it will never be right.