bop11

Has anyone ever pulled the back extension off a 3 speed while the tranny was in the car? Mine seems to have lost the spiral speedometer drive gear from the output shaft (not the driven gear on the cable). I was wondering if I could just drop the drive shaft and pull the extension off to fit a new gear?
Is this the same gear that is used on a 4 speed? I don't see any parts guys offering this gear for 3 speeds.
This is a 67 coupe, original 6 cyl. with the later 3 speed. I understand 66 and earlier 'stangs had a weaker 3 speed transmission If I can't get the gear, I will need to go to one of those GPS sender units as I have an electronic speedometer.
Someone put an '87 302 in this buggy but kept the old transmission.
Thanks for your help.