68 GTS Speedometer doesn't work

When you say it didn't work after the trans was rebuilt, i wonder if the speedometer gear holder, or whatever it is called, is not clocked properly. Look at where the speedometer cable attaches to the transmission. You will see a larger holder, adapter thing that the cable screws into. Take out the 7/16 head bolt out. Pull out the adapter. You will see the gear. They are numbered. If it is, for example, a 36,you can put the adapter back in the transmission. There's numbers on the adapter that you would have to rotate the adapter so the line for the number of teeth you have, lines up.