heating issues when idling 340 73 duster

Like 4speed mentioned above, thermostat is often overlooked. Send extra $ and get a good one, test it in a pan of water on your stove with a candy thermometer just to make sure it works before you install it. 180° is what you need. Test or replace the cap. My swinger would act the same as your car. I would drive to a out of town car show and puke coolant out while backing into my car show stall. Not cool. A new cap fixed that issue. The system needs pressure to raise the boiling point past 212°.