66 Dart Gt Idling issues/electrical

What are you using for ignition components, AKA is this a factory single point, aftermarket "whut?" and so on.

Start by checking for shaft play in the dist, look for dirt/ moisture / other in the dist. cap and rotor, and check overall general condition. Check all wires for continuity. "New" does not mean "good". Make CERTAIN all wires/ boots/ connectors and plugs are properly connected. Check the coil wire while you are at it

Check the spark cranking BY USING THE KEY. "Rig" a test gap using a solid core wire out of the coil. Should generate a nice hot blue spark about 3/8 or more typically 1/2" long

Check voltage getting to the coil. Should be at least 10.5 during cranking and "same as battery" during cranking. Running, should run (with engine running fast enough to keep battery at 14V) should be around 10-12V right at the coil