Sounds like a fun running 326 and should be very peppy in a '64 body.
First, end carbs. Are you using the correct tri-power bases that do not have the idle mixture circuits or are you using standard carbs in place of the tri-power end carbs? Did you install throttle shaft bushings or new throttle shafts to make sure there is no play or air being pulled through the shafts?
Seating the throttle plates shut can become a problem during a rebuild. If they are just a hair off, the will not close completely and allow air/fuel to be drawn through. This is not just a problem that has just cropped up, but was a problem since they came out. Pontiac actually put a black coating of some type of sealer on the throttle blades to help seal them better. Many people when rebuilding them would clean this off thinking it was some kind of build up on the plates - and then have sealing problems.
I read an article that stated the best way to ensure that the throttle plates close perfectly tight with no air gaps, is to set them down on a light box and look down the bores checking for any light shining through. This would be best done BEFORE you completely tighten down the 2 small screws that hold the throttle blades to the shaft. Do one bald at a time and test it a bunch of times to make sure it seats and does not bind Once you are sure the blade is where it needs to be, then tighten the screws up. Then do the other throttle blade. Once you are satisfied that there is no light shining through and they close completely, stake the throttle plate screws so they do not back out. Although this is for a Holley carb, the principals for the throttle blades are the same: https://www.jegs.com/InstallationIns...-26-93-100.pdf
Next, there should be no leakage of gas from the end carbs just above idle IF the throttle plates are completely shut AND your float isn't set too high. The 6 pounds of fuel pressure may also be a problem as this was recently discussed. Factory Pontiac fuel pumps ran 2 pounds of pressure up to about 1800 RPM and then increases to around 6 pounds beyond that. It was recommended that a tri-power only needs 3 pounds of fuel pressure - but I feel this is too low with some cars having a bigger cam/jetted carbs and will suck the bowls dry on a good long full throttle blast. So you may need to add a fuel pressure regulator and set it at 5 pounds and see if that is enough. You could actually drop it until you feel the car "nose over" as it loses power on a full throttle run. Then just bump the pressure up another pound. There are also needle and seat assemblies that work better than others due to their design and will hold more pressure than some others that will not.
Your idle screw setting is fine. The general rule I have always gone by is to gently seat the idle mixture screws in and then back off 2 1/2 turns on each. Then back one side out until it runs smooth, run it in until it runs rough, then back out again until it runs smooth and leave it. Then repeat for the other side. I would not be too concerned about the choppy idle as it is most likely the cam specs over the carb settings and you will find a bigger cam needs more fuel to balance out the additional air it is taking in as compared to the factory cam. You cam looks
mild, but with the smaller cubes, it is actually a big cam. Timing/idle RPM's can also play a role in choppy idle so you may want to play around with this to maximize the engine's performance.