To clarify about front spring arch. The relationship I was referring to and thought you were too is the depth and in out travel of the yoke in the transmission. Since SS springs have very little wrap in front, the distance from rear axle to transmission changes very little so the yoke moves in and out very little. The other factor is change in body lift and the axle swing which again is minimal when you consider the thickness of the chord of 4" of travel on a 2' radius.
