Doug 'desertdog' set me on the right track by recommending a GM shop manual. I found a unit repair manual on ebay for 81 light trucks, and it's all there. Also found a rebuild kit from Denver with same part number as another ebay website selling it for twice as much.

Now to my first challenge: when removing the main shaft, the forwardmost sychronizer assembly (3/OD) became separated before I read the statement that these are a 'selected assembly' and are to be kept together in their original positions. I was able to keep the springs and keys(struts) in their original positions, and the clutch gear sleeve has a paint mark, but there is no evidence of a paint mark on any other piece. By my examination of the components, I can't see how the position of the clutch sleeve in relation to its clutch gear makes any difference, but obviously I am missing something critical. Thanks in advance to anyone who can set me straight.

1957 Chevrolet Pickup Truck