['68OrangeSunshine]

I don't know if this has been suggested above.* I didn't plow thru the whole two pages of threads. But when I bought a '67 K/10 Suburban in 1993, the PO had mounted a mirror on the windshield, MY '72-Style. I didn't like the way it bounced around at freeway speeds [4WD w/ BBC] so I was lucky I had a GM mirror mount [salvaged from my Dad's '64 Olds wagon]. I screwed it back on under the windshield header, and the mirror never bounced at speed thereafter.
I know the '72 MY is missing the embossed mount in the center of the header, but if it was me, I'd try an improvised spacer.

*EDIT: I see it has -- about 3 times...
In the SW desert conditions I operate in, the front on the windshield, when parked in unshaded sunlight, will probably hit 150*F, especially if getting back reflections off a silverized bubblewrap windshield shade. That kind of constant, intense heat will break down any bonding agent. The mechanical/physical connection is the only way for a failure proof mirror mount.