It sounds like an interesting problem but... if all it is doing is giving your rear end "castor", then what does it matter? Castor only has an effect on the front wheels because they turn. If the wheels don't turn, then the camber will not change and the castor becomes pointless.
Whatever the case I am looking at my chassis which I just happen to have in pieces in front of me. The inner and outer hinge pins are parallel on mine. If its not on your then I suggest new arms.