I have a 3 spd heavy duty trans. in a 1958 mercury half ton. It drops out of third gear when I go over 50 miles /hour unless I hold down the shift lever. I believe I need a new synchonizer for 2nd and 3rd gear. The part # I have is an old ford #. The synchonizer assembly # is B8QH - 7124 A. Does anyone know where to find one and is there anything else that could be the problem. I have also replaced the input shaft bearing and the output shaft bearing. This improved the problem but it did not fix it. Don