[Richard]

Not sure why anyone has problems with mirrors falling off windshields. Never had it happen even once. Used of the shelf product once and had good result.
Had windshields replaced on several cars with that type of mirror. Including a 72. Go to a glass installer and have them glue on for you. Cost is probably really cheap.
Regarding the previous post. Put my stock front glass in my 72 and the mounting tab is still good to go. So much for breaking down any bonding agent.
Have a 94 Chev with a mirror on the glass as well. Purchased new, never fell off. Glass was replaced about 5 years ago and mirror is fine. Truck has been parked outside here in SoCal since purchased.