Since a 340 wasn't available in a 72' Swinger, it's really your choice? I believe the V8/318 car's were blue. I'd probably choose whatever looked better with your body/engine compartment color.
That's right RANI, 71 and 72 340 Dart Swingers WERE available! only in Canada. I know of about 10 or so of them in Alberta. A friend of mine has a RED, 4-speed, h-code, 72 340 Dart Swinger. It was sold new at CROSSTOWN CHRYSLER, in Edmonton. They were SPECIAL DEALER ORDERED. About 50 or so cars each for 1971 and 72. ONLY IN CANADA......EH.
If you're using the correct orange trap door air cleaner it really stands out nicely against the blue engine and would be correct but what ever your choice post some pics of the way you go.