I think you may be just a bit over carbed. Mighty Demons are usually for radical cam applications with high durations. They are wonderful carbs when tuned right. I have a 428FE with a comp cams 292/.560 (244@.50)), the 750 Mighty Demon is perfect for this kind of set-up. However there are advantages to the Speed Demon if you run a milder cam. Look at the Demon Selection Guide for your cam and other specs:
http://www.barrygrant.com/demon/default.aspx?page=5
I'm not saying that you cannot get the Mighty Demon to work, but you may not be happy with the constant tuning or the lack of a choke.
--Mike