A tale of two Dusters

I think you'll find a wide front spoiler and a ducktail spoiler like the old Mopar Performance ones they used to sell will help a lot. But that is not a air current issue, just lift and downforce for the front and rear.

The fastback body outline creates lift at higher speeds. So the rear end will get light.

Are you going to put some taped rope on the passenger window? I'd think there would be some turbulence from the drip rails. That's why they put a stainless cover on the Daytonas and Superbird from the side of the front glass molding over to the drip rail in front of the vent windows.