I've been searching the yards for years but still haven't found any car with the same size mirror to use as a donor. (excluding other GM's of similar years such as Pontiac/Buick which aren't easily found either)
I've removed mirrors and can tell you that you better be holding a rabbits foot in the other hand.

Everyone I did was glued in and can either help crack the mirror or at the very least, pull off some of the silvering from the back. It seems like the only ones I had luck with was removing the bad ones!
If anyone is lucky enough to find a donor mirror and want to remove the glass, cut the rubber going around it first. That will help greatly!