I was able to remove the glass from my 68 outside mirror by:
1) hold the mirror glass side up in a vise
2) fill the mirror head with lacquer thinner
3) let sit for 24 hours
4) gently pry the glass out
The lacquer thinner really softened the adhesive.
68 L30/M20 Convertible, Ash Gold
N10, F41, N34, Z21, ZJ9, Z87, D55, G80, P01, U17