I am running a 408 stroker build with many of the same parts you are running. I am using an 800cfm Eddie carb though. I found drilling the throttle plates to help a bit with my idle, I used a 1/8" drill bit. Way too much of my transfer slots were exposed at idle, causing it to run bad in gear. You can check this by removing the carb and looking at them with the throttle closed, or checking for any vacuum present at the ported vacuum port at idle speed. Definitely change out those metering rod springs, they have to be fully seated at idle! I am also running vacuum advance connected to the manifold vac. port with a GM points B28 canister as opposed to the crappy one that comes with the MSD. The adjustable ones aren't much better, the curves are not that hot for performance motors. I am also using the black bushing and light blue and silver springs for the mechanical advance. Throttle response is great, it idles excellent and I get decent mileage now too even with the 3.91's and a high stall converter. I run 18* of initial and 36* total mechanical advance. Vacuum advance at cruise bumps it up to 52* total. My car idles at 850 in park and 750 in gear. It also pulls 16" in park and 12" in gear. Hopefully this helps you too, my car used to run horrible before I started playing around with it.
