Classic Edelbrock carb behavior. I tuned and tuned and tuned my Edelbrock 600 on my 340 and never got rid of that off idle stumble. I put a Holley 650 DP on it, sold the Edelbrock and never looked back. Also gained 4 mph in the 1/4 mile.
My advice - get a Holley Street Avenger 750 and you'll...