1968 GTS 340 with Carter AVS 4966S

I agree that its better to work with the factory non-electric arrangement if you can.
Tapping into the ignition circuit adds a big load on circuit that was never made for that.
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For both idle speed and timing a tachometer will be very helpful.
Looking in the '71 service manual the 4966 on a 440 was supposed to idle at 750 rpm with an AFR of 14.2:1. The idle mix screws worked in the normal fashion but had plastic caps on them to limit the adjustment range.

A '68 340 was was to idle at 650 (automatic) or 700 rpm (manual)

There's no reason for you to run the engine that lean. So you can experiment a little with the idle rpm and fuel mix that produces the most power. With an automatic that's easy to check by placing the transmission into gear and observing rpm or vacuum drop. The least is best.

Keep in mind that backing off the idle speed screw is not only reducing the airflow. It is also reducing the fuel contribution and increasing the air from the transfer port (slot). So if the throttle is slightly closed, and the idle mix screws aren't touched, the AFR will get slightly leaner.