As long as the the clutch is released and the transmission is in gear, you can loosen/tighten the bolt holding on the u-joint. In the video he didn’t have the engine to keep the tranny from turning.
The speedo gear on the tranny tail shaft is a slip fit. It is held in place by the u-joint when its tightened to the tail shaft. There is a spacer that goes on before the speedo gear.
The hardest part is getting the torque tube dropped enough to get the u-joint removed. Changing the gear if required is easy.
Here is the tranny breakdown from the
Chevrolet Truck Shop Manual.
