My 340 Flattens Out Above 55 MPH

My 340 flattens out above 55 mph and it won't pick up any more speed and I can't keep up with traffic on the highway. I don't even know where to start to look. Plus it idles rough at a stop and in park but it picks up speed and cruises at lower speeds ok. Is it the carburetor (avs), or ignition, too much timing...??? I can only fiddle with it for so long then I get frustrated. Once again, I'm lost. There are so many smart people on here that I hope can give me a clue.

It's a 69 340 Swinger, electronic ignition, avs, automatic, 3.23 gears. It's my "It used to run fine and now it doesn't" story.

Thanks very much!