Spring relocation kit?

So is it necesarry to move your spring perches if you choose to use the offset spring hangers?? Since I am going to try to put in a b-body rear I'm going to have to move them anyway....I would like to fit at least p255 or P275's along with a decent slick...

I have a set of 15x8 with 5 1/2 backspace "Welds" and a b-body 8 3/4 I was going to put in my 67 cuda.

jim