D45 mirror reinforcement arm/bracket

[K25alive]

Looking for feedback regarding these mirror supports. They are used to reinforce the lower mounting bolt on the D45 below-eyeline mirrors. I pulled these off two '88 Suburbans. Note the shorter/smaller one came off the driver door. Are these supposed to be two different sizes?
I'm going to remove the camper mirrors and add the D45 mirrors to my '79 K25 truck. So another thing, how would one know precisely where to mount the mirrors on the door so the reinforcement arm will line up? Is there a diagram available out there?