I moved the rear end back 1" on my 69 Dart. I had the same question as you so I called Calvert. I was told that moving the spring hanger back will affect the rear shackle geometry and that it would be best to do it with the spring. So that's what I did, ordered springs with the rear set back 1". The spring isn't any longer they just move the rear end housing locating pin back the 1" to achieve that.
I've already got the rear shackle relocated to the frame and an inch back, so I can either move the front eye and inch back, or order springs with a 1" longer front segment. I'm wondering if a longer front segment is better or worse, or negligible. I always thought it was best to have the stiffest front segment possible, which would mean shorter, right?
