heating issues when idling 340 73 duster

All of the suggestions could solve your problem but it's clear you have an air flow issue. That fan is not pulling enough air through the radiator. I'd like to see the design of the shroud.
In the mean time, take a look at the following;
1. is there any obstructions in front of the radiator that prevent the fan from pulling the air through?
2. Does the shroud cover the entire core?
3. Is the shroud sealed to the radiator around the perimeters? That will create a suction affect and increase cfm.
4. is your WP pulley the same or smaller than your Crank pulley? You want your WP pulley to have the maximum RPM's.
5. Tell us more about the radiator. ie; core thickness and amount of tubes.