The chart I have shows your stock axle would be 51.4" flange to flange and 43" spring width. A b-body is 55" F to F and 44" spring width. Looks like you will need to change the drive shaft also.
Please kill that chart, it's horrible and wrong on a bunch of measurements.
A much better reference...
http://www.moparts.org/Tech/Archive/axle/16.htmlStock 68-70 B should be 60.125ish wheel mount to wheel mount. 44" perches are 1" wider than an A at 43". Lots of people pull the springs to get B rears installed.
It may fit OK. Wheels tend to need a bit more backspace than the stock mopar wheels with a 68-70 B rear in an A body. Dusters are more forgiving than Darts.
