No need to disturb the dash, the work is all within the instrument cluster wood surround. Take out the clock first.
Remove the lower dashboard panel, reach up from below and unscrew two knurled nuts, rotate clock about 1/8 turn (it will only rotate one way) and remove to the interior of the car. Do not remove into the dash. You may find that removing the steering wheel makes this easier since it gets in the way.
Once the clock is out, the speedo is more accessible through the clock hole. Draw a diagram of the wires connecting to the speedo head, then you can reinstall once and it will be correct.
I had a spare instrument cluster to practice on, so it was not difficult once I decided to do it. There are photos posted and if you use the Search function you should find them. This topic has been addressed several times.