Due to the size of your cam, it sounds like the butterflys are open beyond the range of the idling circuits. You can drill a small hole in the front half of the primary butterflys ( I use 3/16" or less ). This will allow you to close the butterflies, but still get air. I found this tip in "How to Hot Rod a SBC".

I used this trick on a roller cammed BBC and a SBC. Worked like a charm and improved idle quality.