How to center rear wheels in weel well

I have read a lot about how to center the wheels in the rear of my 1972 Plymouth Duster. I have the springs moved inward and the current size of the tire are 275/60/15. The trouble I have is the passenger tire sits a little closer to the rear of the opening than the drivers side. I read you could re-drill the hole in the spring perches to adjust for this or fix the box mounted to the front of the spring. I will be taking the car over to mans house that could do what ever it takes to fix this trouble, but just wanted to tell him what you thought would be the correct way. I do not race it nor launch the car hard by no means. Just the best way to repair this trouble. I am not sure how the front mounts are mounted but I can find out more if you ask me what to look for. If there is no answer now how to resolve this issue maybe I should buy a 255/60/15 tire and be done with it.

Richard Jorgensen