Transfer slot Holley 1 bbl

make sure the curb-idle screw is on the throttle stop; and the fast-idle cam is not interfering.

But yeah, in the pic you can see carbon/fuel-wash evidence that this particular carb has been running in several different idle positions for extended periods of time.
Make sure that ;
1) the cylinder pressure is adequate and even, and the valve-lash is reasonably close, and;
2) the Ignition system is firing all plugs, the timing systems(plural) are all working, and;
3) the PCV system is functioning correctly, and;
4) the fuel is fresh and at the correct fuel level, the low-speed fuel delivery systems are clean and working, the choke-system is functioning correctly, and that the engine is not getting air from somewhere besides the throttle, and that the system is vented at both ends, ie the tank and the float-bowl, but that none of the rubber jumpers are sucking air.
5) that the heat control-valve in the exhaust manifold is functioning correctly.
6) if you have installed a later air-filter housing with the heated air intake, and plan to run the car at colder outside temps, then the heated air intake system, should be working. If you have installed the fresh-air intake piping on it, then doubly so.

Those systems and conditions all work together at idle, and if any one of them is working poorly or not at all, almost invariably, the throttle position is adjusted, in compensation.

If the intake is aluminum, then the carb should be on studs as the threads will last longer.
If the intake is iron, then, IMO, it doesn't matter, except, if they are studs, I find it easier and quicker to install the carb.
If the vacuum advance system has any delay system on it, I would defeat it, but;
If this leads to detonation, then put the delay mechanism back on until you learn how to overcome the detonation.
If your PCV hose is regular fuel line, then under high vacuum, as the hose gets hotter/softer, there is a good chance that it will collapse and fail to work. When this happens at speed, pressure will build up in the crankcase, and return to the intake side, and/or blow out a gasket or a seal. The pressure always finds it's way out.
Proper PCV hose is a thick-walled hard tube.