11:1 is pushing it for pump gas. You need more initial timing, start with about 16 - 18, vacuum disconnected and plugged, and maximum 38 degrees, all in by 2500 or 3k. If you have detonation pulling a load, back total timing down in 2* increments until it goes away. Have you checked all around for a vacuum leak? You're definitely running rich, 800 cfm is a lot for a street 383, but doable with a good tune. I have the same heads on my Nova, using the same plugs. I used 3933's initially, but now use RC12's as well. Short drives around the neighborhood can be a problem with fouled plugs.
Slow typing, BBR mentioned some of these points too...