Door mirror brackets on 70 c-10

[ktztrk]

I have mirrors that are said to be for 67-69 chevy trucks that have a triangular mounting flange that matches the flange on my truck doors. There are three holes at the points of the triangle and a center hole. On my truck, there is a bracket on each door that matches the mirrors.
Is there a re-enforcing piece behind the bracket and/or do I have to remove the bracket to install the mirrors? I have taken a pic of the bracket, but will have to do research to learn how to upload pics.

I have tried to upload from my pc to the board, a jpeg file that shows the brackets. It is shown as an attachment in the additional options section as olshotgun2.jpg.
Any help is appreciated.