Back on the road!!!..... for about 5 min (engine help please)

My 69 Dart w/ 225 ran like that for decades. I had several mechanics look at it. Most said "bad valves", but didn't do a compression test that I saw. I tried many things, including valve jobs, different intake, several rebuilt carbs, electronic ignition (Crane XR700), even a new long block. Same "lean idle" shaking and slowing way down in D or R like yours, even stalling when you give it a little gas. Not fun to stall when a light turns green. Finally, the 4th Holley 1920 carb cured it, and it ran perfectly.

I know a lot more about cars today and have more diagnostic tools. One thing I learned is to not waste money on mechanics. If you have a Holley 1920, I think there are many poorly rebuilt ones out there. The sealed metering block can't be rebuilt or inspected. One guy here reported luck blowing air backwards to clean out the air metering circuit. You could also try a Carter BBD or Holley 1945. The BBD is simple to rebuild and adjust, the 1945 is preferred by many. If you close the choke plate slightly and it idles better, that is a sign of running lean. Certainly change to electronic ignition (HEI module, search here), since that will let if fire better at leaner conditions, giving better running and economy.