Keep in mind that the 68-70 B body axle is the widest of the B body axles until 1971. The Demon, Duster and Dart Sport have rear wheel arches up higher on the bodyline, allowing more tire clearance than a standard sedan/hardtop Dart or Valiant. Comparing axle fitment on a Demon VS a Dart isnt completely fair. There is a spec chart of axle widths on this site. Check it out.