My dad owned an auto salvage for 30 years beginning in 1960, the prime years for stovebolt parts selling! One of his good selling parts was the 3 speed transmissions for Chevys. We just couldn't keep enough good used ones so he began to repair and rebuild the exchange units. He did literally hundreds! In addition to the synchro, the main drive gear was the biggest culprit in the jumping out of high gear problem. The splines that engage the synchro become worn and 'push' on the syncro ring causing it to jump out of gear.
One trick all the trans shops used back then, was to grind a very slight curve into the side of each spline facing the direction of rotation. This was done freehand with a stone in a die grinder or drill. The stone was about 2" in diameter to cut a long curve. The object was to reverse the push created by the worn spline, and pull the synchro ring into, not out, of gear.
If you open it up to replace parts, examing the Main Drive Gear closely, it it shows any wear at all in this area either change it or try the trick above.
