I ended up removing all the crews on the back of my speedo when I replaced my odometer gear on my 84. I seem to remember that I could have done it with only removing 4 but is was no problem getting all 6 back in. Just make sure you do not pinch any wires.
If you search the archive there is a reference to removing the odometer reset push rod and then rotating the face so the hole is over the 4 screws so you can remove them. I tired it but could not remove the push rod. I ended up snapping the needle off. Just sent it in for a $170 repair!
Here is what the speedo looks like with the face off.
When you replace the odometer gear you will have to remove two or three additional gears and then reassemble in the reverse order.
Good luck.