box not straight

[lumpyguy]

I have a 1969 gmc long box I have re build almost everything but I have the box on and the left rear is an 1 inch lower than the right so when you look at the rear the turck does not look straight. I have checked everything and all looks straight with a tape measure the springs are 4 inch drop and the same size and thickness left to right I even swaped the spring and still the same is there any shims that can be put under or over the spring to stiffen and lift the left rear or would it be best to drop the right to match any help would be great thanks