1966 c10 speedo help

[Zcle91]

this is my first time posting so if im doing anything let me know. i have a 1966 c 10 as the title says my speedo is not working i tried the "drill" test on the cable and it works then. i looked at the gear that the cable attached to and it is in good condition the gears look sharp. when i connect the cable to the gear through the metal "plug" and spin the gear you can feel tension so i know the gear is connected to the cable. i put my finger in the whole where the plug goes and i can feel the gear around the transmission. but when i drive nothing happens. so im guessing my next step is taking off the end of the transmission to see if that gear is free spinning on the trans?to do that i have read that you need to remove the drive shaft? i will do that but my driveshaft center support is riveted to the frame. so before i start grinding rivets off next weekend i was wondering if anyone knows a different way to take off that back case or a step i am missing through the whole process. im very welcome to any help at all thank you all