Slant Six 225

Hi my name is Brandon and I am having so problems with my 1969 Plymouth Valiant 100 slant six 225. The car runs fine in Park when you put in Drive or Reverse the car starts to shake real bad as soon as you put a load on it at a stop but as soon as you drive and get going it smooths out with no problems. I have replace the plugs, plug wires, carburetor, and put on the pertronix electronic ignition to get rid of the points and still has this problem does anyone have an idea to this problem or have had this problem with there slant six?