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.