Back spacing on B body rear end in 73 Duster

Well its stock spring location and and I did move the perches in to stock width. I guess what I would really like to know is I want to run the ralley wheels but they don't come in more than 41/2 backspaceing so I looked into having my 14's changed to 15 X 8 and price wise it is 130 a rim and they can move the backspaceing but I did find some 15 X 8 for 200 a pair but still only 4.5 it's just I have heard it both ways to move out and not to move out.