If I understand your situation, I ran into the same problem adapting some Ford Torino mirrors to a '65 Corvair Monza I had a number of years ago.

I, ah .... appropriated some blocks of 1/4 inch thick wood-epoxy type material from my employer and sanded them down at an angle to make kind of a flat wedge. It took a little trial and error to get the angle correct so that when I held the "wedge" on the right place on the door, then held the base of the mirror against the wedge, the mirror was at the correct angle to the door.

Then I drew around the base of the mirror and cut the wedge to the shape of the mounting surface of the mirror (actually, just a little larger). Whatever the material was, it sanded and painted nicely along with the mirrors. When the mirrors were bolted down they looked surprisingly good. The car actually won a trophy or two.

Did this make sense?

Anyway, total cost of the project, Zero (unless you want to count a spritz or two of rattle-can touch up paint).

Jim