If you are only going to remove the speedometer, I wouldn't worry about removing the whole cluster and messing with the vaccuum line for the boost gauge. Just take the lens off the cluster and then pull out the speedometer.
Also, I usually remove the trim around the steering wheel before I remove the trim piece on the dash, although I don't know if you have to do that--it's just easy to do on an early model. Be careful when removeing the trim piece as there are spring clips that hold most of it to the dash (and the 2 screws that are under the top lip of the dash). I managed to break a few of the tabs that hold the spring clips during the numerous removals over the years.
You will also need a little screwdriver to remove the headlight switch knob. Pull the knob out and look for a slit. If you look carefully, there is a piece of metal that goes across the slit (part of another type of spring clip). Use the screwdriver to push the clip away from the end of the knob, while lightly pulling the knob. The knob will slide off. Now turn off your headlights.
Of course the above information is for early model SC's and I didn't look to see if you listed the year of your car before I typed all of the info.