heating issues when idling 340 73 duster

You didn't say anything about any mods done to your 1973 lo-compression 340, but you also didn't say it is stock.
As others have said, you have an air flow problem, or a waterflow problem.
Or you are running retarded timing.
I would build the cooling system up to overkill, and once it's working properly, then you can start cutting corners.
So do all the things already mentioned.And I would add;
1) do a compression test to make sure none are blowing into the cooling system
2) get your D recurved to 20* in the D and set Idle timing to 14*, and the all-in (no v-can) to come in no sooner than 3400.
3) If that clutch is viscous only, I highly recommend to get rid of it. Overkill is a large diameter , hi-efficiency,all-steel,direct-drive, 7-blade fan, stuck into a close fitting shroud, half in, as mentioned. Second best is to put a Thermostatic clutch on it.
4) If you are still running a heater, let it be your bypass, and restrict the crap out of the other one. If not running a heater,install a hi-flow stat , and let a lil bit of bypass thru.
You absolutely need an anti-cavitation plate on your pump vanes.
5) If your carb is sucking hot under-hood air, at idle and stopped; that would be operating like a runaway nuclear reactor.
6) You don't need a million-dollar radiator. Once everything else is working right, the rad just needs to be clean inside. Mine is a ancient 1973, out of an air-conditioned 318 Dart.

Once you get it working, I would bring the minimum coolant temp up closer to 190, or more.
Happy HotRodding