Just what you guys wanted, another weenie asking about b body rear in a dart swinger

I don't see a problem doing this swap as long as you aren't concerned about super wide tires and are willing to get custom wheels and tires to fit comfortably in the wheel well.
A friend of mine has that exact set up you were describing in his '68 Dart and doesn't have a problem with it.
I say go for it and after it's installed, measure for the wheel offset you need and go from there.