Cooling issues 69 Dart GTS 340

You have both an idling issue and a highway issue. Neither is real bad.

Is the 340 stock or has it been rebuilt? Any performance upgrades? The 2 core should be fine if totally stock. The suggestions above of a shroud and a 15lb+ cap will help for sure. If that doesn't do it and you've done some upgrades, that might be pushing an oem 2 core to it's limit.