Duster VS Dart

Let's also note that Dusters/Demons have a fixed "B" pillar, which should theoretically make them stiffer than a Dart hardtop.

I for one do not by that "1 inch longer wheelbase" Dodge vs Plymouth.
I think that was a marketing gimmick designed to up-sell the Dodge line.
Think about it- same floor pan, same frame rails, same springs.....where's the inch?

If you've even hung springs or an axle, you'll also find that about an inch is within the tolerance of those parts fitting together.