A couple of simple tests to start:
- Put a vacuum gauge on the intake and measure vacuum levels. I would expect it to be up above 15 in on this setup, and very likely around 18". This sounds very much like a vacuum leak in the intake area or under the carb, and this test will show that.
- While running, pull the PCV out of the valve cover, still connnected to the hose and put your finger over the open end of the PCV. See if this helps the idle. If it is hung open at the cruise opening, then too much air will be pulled in at idle.
- Use a piston stop to find exact TDC to make sure your damper ring has not slipped.
- What is the points gap set to? It ought to be around .018" or so. A too-small gap will cause this exact symptom. Also, inspect the 'nubs' on the distributor (that operate the points' rubbing block) and make sure they are not worn down. If so, then the dwell (the time for the points to be closed to charge the coil before each spark) can never be set properly. This also causes this symptom.
- Did you put in a new condensor in the distributor?