Idle screw question. Rich or lean?

I think neither; There's a slim chance that the fuel level is a tad low. But
I think you need to put the throttle blades a little further up on the transfer slots, by increasing the curb-idle.

To me; I agree with your assessment that the throttles are closing to far too fast and a momentary leanness as the mixture screws are forced to take up the slack. You may have to back off the idle-timing if the rpm goes too high, and that would require a revisit to the power timing.

IMO
with a V8 small-block stick car, there is no benefit to a bunch of idle timing. In fact, it just makes the engine too powerful at low-rpm/very low road-speed ,and then it becomes too difficult to drive very slowly without slipping the clutch.
With 19 inches of idle vacuum, you can sacrifice some of that to get a better slow-speed driveability.
Try sacrificing 4 to 6* of timing and, increasing the curb-idle screw at least 1 to 2 turns., and see how she likes it. The mixture screws should end up between 1/2 to 3/4 turns out from lightly seated.
Sooner or later she's gonna want a 10.5PV.
Good luck!