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 be happy camper.
Regarding the comment about disconnecting the vacuum advance - bad idea on street car. Get an adjustable vac advance. You may need to play with the initial, the total and the adjustment of the vacuum advance to get things right. But removing the vac advance should only be done on racecars.
The Feb Mopar Action has a great article about vacuum advance - why you need it and how to adjust it.
