I did this a while ago, but the speedo drive is currently in my transmission, and my memories are a bit faded. But as I recall, you have to remove the speedo drive housing (it's held in place by a retainer plate and a bolt on the later, post '70 models, or by a retainer pin on earlier, pre-'71 models). The plastic drive gear then pulls out of the housing; it sticks a bit, so you have to give it a bit of a yank. Then you can push out the cup seal from inside of the housing, and install a new one (they are available from OGTS). I recall asking Gil if mine was installed correctly, because the cup stays stationary in the drive housing and seals against the spinning plastic drive gear shaft, which is opposite of how most cup seals function. But he said that is how it works. I believe he also said that you "could" install the new cup seal (cup towards the transmission) on top of the OLD seal, so that you now had two seals, and it will be only partially inside the drive housing. Then the speedo cable end holds it in place when it is attached.
HTH
HTH
