A 66-70 B-Body housing is 3/4" wider overall than the stock axle. Cut the perches off and move them (you should do this any way to get the pinion angle correct).While you're at it it's easy to move the springs in(search my old posts for pics). Duster or Demon drive shaft from a 727 equipped car fits,as does the 727 itself.B-body headers usually fit with some issues at the steering coupler which can be trimmed. A last note. I haven't done it myself but a friend claims if you swap the stock 318 motor mount brackets right to left the R/B engine will come very close to bolting in.
Good Luck
Doug