63 Dart overheating issue

Typically a cooling system clean out puts a "T" in one of the heater lines.

The hose pushes water into the block than back out through the radiator and out thefiller neck.



Sounds like you have done most everything cooling system related.

Overheating at idle is typically an air flow issue, overheating at speed is typically a water flow problem.

Have you checked timming at 2000 rpm?

Maybe your mechanical advance is stuck.

Also check your vacume advance, the vacuume at the can, it should be 0 at idle and much higher (10 to 20) at 2000 rpm in park/neutral.

If it is hooked up to a manifold vac port it will add advance at idle and might subtract timming at speed.

Just reaching for straws