I have no working speedometer

Something is moving while driving. Or is not fully engaging. There is a plastic gear on the final drive housing that may not be staying where it should. This engages another gear that is ion the shaft inside the small housing that is bloted to the transmission housing. This small gear is driven by the gear on the final drive unit, and in turn drives another gear on the top of the shaft. This gear is located in place by a thrust bearing and a roll pin. The gear on the top of this shaft is the one that drives the speedometer gear and speed sensor. One of these gears is not staying in place, or is not fully engaging. This could be a missing part (roll pin or thrust washer) or incorrectly installed part.

The only thing that would require major disassembly is the gear on the final drive assembly. Everything else can be done without removing the transmission. A good start would be to get the ATSG manual on your transmission. There is a complete breakdown and illustration of the components that make up the speedometer drive.