I don't know about your 38, but I did disasemble a '36 speedometer years ago. My memory is that it was the same design as a 60's era speedometer that I took apart to see what they looked like.
You might try disconnecting it at the transmission to see if you can pull the cable out from there. Then, provided the nut has been removed from the speedometer, you should be able to wiggle the cable housing loose from the speedometer.
Just a guess, hope it helps...
