I am not sure if you are trying to replace the entire speedometer housing and all, or just the inner gauge or head as you call it inside the housing. Anyway, here are my thoughts on each scenario.
It is not too hard to remove and replace the entire gauge if you have done it once before. Unfortunately, if it is your first time remove it, it can be frustrating. You can't see or hardly reach the nut at the 1:00 position when looking at the gauge from behind. It pretty much has to be done by feel. Other than that pull out the gauge lights, and after the four nuts are removed it should be able to be pulled out. I believe there are tech write-ups on this already, so this may be something you already know.
If you are talking about the gauge inside the housing, just pry up the chrome trim ring where it is pressed down around the housing, remove the two screws from the back of the housing and the gauge should come out.
I would be curious if you know what caused the other speedometers to fail?
I just noticed your signature line, if this is for your 64 you can disregard what I said above. It may be worth posting in the 60-66 section if that is the case.
