I've had the mirror glass replaced before. When I was in high school and car alarms were still sort of a new thing, some jerk busted my 81 2wd Jimmy's mirror while at a bush party just to hear my alarm go off. I had a local glass shop replace the mirror. They had template and everything, so they did a few.
I always thought the heads were pressed onto the arms and they weren't really supposed to be removable. I 'removed' one inadvertantly once (again in high school but in a different truck, an '80 longbox) when I was bombing through a snow-filled alleyway and got a little too close to a truck equipped with a flatbed...... Took the mirror head right off.