Normally I would say the 68-70 B rear is a little wide for a Dart, the 65-67 works a bit better. But, if you're pushing the quarters out an inch you'll have the quarters in pretty much the same place as my Duster does, so it would work pretty well. The 65-67 B body 8 3/4 is 59.5" drum to drum, the 68-70 is 60.125".
Guess you stretched the front fenders a bit too? I know Tom was running 285's on his Valiant but he pushed them out with a porta-power to do it. :D