340 Chugging instead of Reving

The carb responds to the pressure differentials it sees from atmospheric pressure above it, to the reduced pressure (commonly called vacuum) below it. It doesn't care about rpm.
So if you have a lot of Idle-vacuum, the engine will pull hard on the Transfer Slots and Idle Discharge ports which are under the throttle-valves, which should be nearly-closed.
But if your throttle is too far open because of retarded timing, then the transfers will be dumping because the fuel flow is directly governed by the amount of slot exposure; you can't be using the Idle Mixture screws to band-aid too-far closed throttle valves. If the engine won't idle right with the mixture screws at or near the center of their adjustment range, something is wrong, and my bet is that it has to do with the T-Slot exposure..
But if you have a bit of a cam, then the vacuum will be lower to very low, easing the tug on the Transfers.
But
The worst thing is that the boosters don't care which way the air is going thru them, in order to respond. So if your intake valves are not closing 100% while the boosters are on line(*1), and the plenum pressure gets to pulsing, that pressure-differential pulsing may be read by the boosters, to which the carb will respond. If the pulses actually reach the boosters, then there's gonna be even more trouble.

(*1) or there is any change of air-pressure in the intake plenum; like a mini explosion/"backfire"; or as when the overlap cycle gets to pulling hard.
(*2) some air-filter housings wreak havoc with bowl-venting
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As to rotor phasing; this is easy to prove. Just put a timing light on it and rev it up while watching the timing mark;
If nothing crazy happens then you don't have a rotor-phasing problem.
If the strobe starts missing when the rpm gets into the chugging range, but the timing marks do not otherwise change, probably the spark is being shorted by excessive fuel; Put a vacuum gauge on it and see what the manifold pressure is doing. Or, possibly, the coil is failing. Between the vacuum gauge, and the strobe-action, something should give you an idea of what's going on.
If nothing is talking to you, check the valve timing.
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I don't see a vacuum advance. For a streeter this is as good as crazy; Your timing will be retarded almost all the time, and the engine will be very fuel-thirsty while on the low-speed circuits..