Stuart,
I do not know positive from negative caster (1 +/- 1/2 degrees is the setting); however, every Advance-Design axle that I have seen has wedges between the axle and spring with the high part of the wedge dowards the rear edge of the axle.
You want backward tilt of the axle and king pin (Shop Manual); and, the springs are on top of the axle. The wedge tilts the rear edge of the axle downwards - is that backwards?
"To increase caster, place the thick side of the shim toward the back between the spring seat and the spring." (Shop Manual)
Tim
