i just measured my 82 el camino with the round sport mirrors it;s 13 3/4 inches from the front of the door to the front of the mirror and 1 1/2 inches from the body line to the center of the mirror and yes it needs to be reinforced from the back side one car i had i made a plate threaded it bolted it on through predrilled holes in the door and drilled 2 holes through the door into the plate and pop riveted it in place