On glow fuel Ignition and some very careful carb adjustments, we have had our 320 idling in the 400-450 rpm range. Its running so slow and with so little fuel that even with the factory straight pipes its almost silent. Folks keep looking to "find" the electric motor.
700-800 rpm is a good flight idle, the 400 is just to impress people.
If you have trouble getting it to run correct check all the Orings in the intake system.