318 Engine stumble

It sounds like a good diagnosis. Raise the float level a bit and retune the idle mixture and then see if issue gets better. A higher float level generally generally will make the transition circuit richer. Also, look at the top area around the primary bores, and look at the air bleed there are 2 on each side. Make sure that none of the air bleeds have been drilled open. If the idle air bleed has been drilled out, it will lean out the transition circuit.

Also I am a bit baffled why the accelerator pump does nothing when opening the throttle slowly. When you say check valve, have you checked the little weight and check ball under the pump nuzzle itself? When not running, just a slight movement of the throttle off of the idle stop ought to produce a dribble from the nozzles.

BTW, what Holley List # do you have?

The engine is likely at it's leanest when this occurs, and the mixture may be harder to fire. So, if the ignition still has these odd dwell issues, it may be time IMHO to put in the other system and see if that improves this.