If your vacuum is dropping alot at in gear idle it's probably trying to pull more fuel and that's killing it. The fact that new plugs helped would seem to support that theory. A true vacuum reading in and out of gear would help the diagnosis, and give us a measure of how much cam you really have. If your considering changing carbs. i'm curious what intake you are running. Do you know your compression ratio? I'm biased on the subject, and not sure how much cam you have, but big, meaning 240 deg. @ .050 or more are much more managable with a Holley carb, jmo.