The alignment specifications in the truck manual I've got handy at the moment only go back to 74, which is a little later model than your system. I have a feeling the tread wear problem will not be correctable, due to the major change in the scrub radius caused by the change in the spindle position relative to the angle of the control arms. There is no correction available for the error you have created by lowering. The relationship between the ball joints and the tire centerline has been changed in such a way that the factory-engineered scrub radius has been permanently altered. The best you can hope for is that the vehicle will be safe to drive, and won't have unpredictable and potentially deadly handling characteristics in an unexpected sharp turn or accident-avoidance maneuver. It can probably be made to track pretty straight as long as nothing dramatic happens. Good luck!
Jerry
