The speedo adapter on the trans just screws out. Assuming you're talking about a 3-speed tranny, the speedo gear in the tranny slips on the shaft, followed by a sleeve and then the u-joint yoke, held on by a bolt that prevents the gear from slipping on the shaft.
I have my doubts that the problem is in the tranny. You can check to see if the adapter is working once the driveshaft u-joint is disconnected from the torque tube. Put the tranny in neutral and turn the u-joint and watch for motion at the speedo adapter.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.