Dart over heating???

But as it is over heating at idle,

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Most of the over heating at idle problems I've seen or heard about were related to flow issues; either water or air.

If this car is stock, it didn't come broke from the factory and that's with no shroud.

Try washing the radiator exterior real good. Spray it down with Simple Green or good engine cleaner and hose the heck out of it. Be careful not to damage the fins with pressure or jetted spray. Wash from engine side forward. What does the inside look like?

It doesn't happen often, but if it set for long times, it could be the water pump's impeller or pump casing is rotted enough that at low flows it can't move enough water.

I don't think your fan spacing is too much. You can lose more efficiency being too close than a couple of inches away. If the fan is a couple of inches smaller than the shroud, that's only an inch or so on each side. The important thing is it should be IN the shroud.