Dart with b body 8.75

Which B-body rear end? There's like 5 different widths...

An accurate 8 3/4" rear axle width list

Regardless though, I can pretty much guarantee a 15x8 with a 4" backspace won't fit regardless of which B-body rear axle you have. That wouldn't be a good fit even if you had an A-body 8 3/4, you need more backspacing.
I have a '68 Coronet B body 8.75 axle in my 67 Dart. 15x8 wheels do fit but they had 6" of backspace. Custom wheels from Wheel Vintiques. Blue Dart.JPG