I have a 67 power top that when it is clamped down the joint behind the clamp drops down about 1/2 inch. This stops the window from winding completely up. When clamped there is a 1/4 inch gap between the clamp and the truss joint above the driver's seat back. Not sure how to fix. Little info here in Australia. Have adjusted the truss head screw but nothing happens. HELP. Prepared to buy any/all parts to fix as I am getting wet when it rains!!