In addition to the above, did you ever properly set timing? Did you just set the idle screw, or did you actually adjust it? That is, for best / highest speed / highest vacuum
Maybe you have a dead cylinder or even a bad plug. One way to check this out is to "rig" a grounded probe, and pull the dist. boots up away. Sometimes, if you don't have a nice tiny probe, you can slip a small brad down beside each plug wire. Then, with the engine idling, go around the cap grounding one plug at a time. Listen to the engine and get a tach on it if possible (Dwell tach meter) Each cylinder should drop RPM about the same.
Does it idle smooth? Any reason to suspect a vacuum leak? You have a power booster?