Look at the sport mirrors found on the 72 Lemans and other Pontiac cars. They were used for many years after that on Pontiac, Buick, Olds and Chevy. I have them on my 79 TA and they may be the same style as those from 72. They have a bracket and plastic seal/gasket that is placed between the bracket and the base of the mirror. The bracket is fastened to the inner frame of the door structure with two tapping screws. One threaded screw mounts the mirror to the bracket.
Hope this helps.
"Dedicated to keeping the classic Pontiac engine alive."
Yes, i had read that when reading an article in smoke signals recently about the first design of what became the gt-37. The engineer ordered them and some other equip, and they canceled because the vent window hit them. Have to see if i can find it.
I looked at doing this with my 70 years ago and I seem to remember the F-body sport mirrors having a different angle at the bottom but I don't remember if an adaptor standoff/"gasket" existed to correct it. Also, the mounting differs too as the F car mirrors typically mount on a pre installed plate/bracket and attach with a single Philips screw while the A-body mirrors can have the two studs that are retained by nuts under the door skin.
Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.