Truckernix, that's really odd, as my spacers were installed on the outside of the rear springs on my 1950 3600.... and you can't move them inboard without changing the location of the spring perch on the axle, and I am sure that I had a bone stock rear end. The spacer moves each rear spring 1/2 inch, which would be noticed the moment you tried to bolt both leaf springs up!
During my restoration, I actually moved my spacers inboard when I swapped a different axle in, to give me more clearance between a new heavy duty spring plate and the frame (I flipped my U-bolts around so that the curved part went around the bottom of the axle and the spring plate was on top of the spring).
