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I took mine out of my 77 a couple of times when I had a wiring problem. It will pull off. Sometimes it's a pain. I got mad and gave it a good yank once and had to replace it yesterday.
You don't need to remove any bolts to detach the speedo cable, There is a plastic collar on the end of the speedo cable, you need to push it towards the center of the cable and then pull on the cable. Maybe someone here has a picture from a manual that they can show you?
Mine was so contrary that I had to get a pair of channel-locks and open them wide, then gently squeeze that white plastic clip to get the cable to come loose. Point the opening of the wrench down, you have to squeeze it along a horizontal line, if that makes sense. At least I did. The manual tells it like, "Just give it a gentle squeeze and POP! it'll come right off." Right. Don't get too frustrated, just keep workin' at it. Gentle squeeze with some wide-mouth pliers ought to do it.
Just give it a gentle pull, or if you really get miffed, buy a new speedo cable ahead of time. Like I said it was a bugger to get off then a good heart-felt yank took care of the problem!