493 carburetor tuning question

Scampy is correct. Transfer slot relationship at idle is very important. A portion of the fuel at idle comes through the transfer slot. That portion is not controlled by the idle mix screw. It is only controlled by the amount of slot exposed to atmostpheric pressure vs the amount of slot at low pressure.

Go here and scroll down to 1966 for an overview of carb fundementals. Theres some slight differences between Holley and Carter but the main one to be aware of when reading or watching that Master Tech session is Holley uses a 'power valve' for the wide open throttle enrichment. They also skip over the role of transfer slot at idle when they are first introducing how the idle system works. Later they touch on it.
Master Technician Service Conference - Chrysler's Training for Mechanics

Second. Timing and mixtures are closely related. Even with highcompression, that cam is probably resulting in a fairly inefficient idle.
Timing at 1000 rpm probably should be around 22 - 30 deg.
I wouldn't try to use the vacuum advance at this phase of tuning.
Later, much later, you can work on using it to bring the timing while cruising (at 2800+ rpm) to something around 50 to 56 BTC.

here's a guide to ballpark the initial timing.
Distributor starting point for a curve

Depending on high rpm efficency, the maximum timing around torque peak shouldn't be more than 36 or 38 degrees, give or take. Too much at wide open throttle full load can cause damage.

Post here summarizing the concept of timing the spark.
Large RPM and Vacuum drop when shifting from park to gear