I don't think the movement of the shackle will cause you any problems, because the fore/aft position of the axle isn't going to change. The shackle simply lets the spring stretch or shrink in overall length as the axle moves in bump/rebound. The radius from the spring pivot to the centerline of the axle shouldn't change enough to induce a noticeable amount of bump steer, regardless of whether the steering gear is at the front or rear, IMHO.
Jerry
