Do not attempt to bend the mirror arm , it is of cast material and will break.
Not the one I have. I believe it's a steel forging. If concerned about breaking, use some heat.
You have the wrong mirror arm.
The trucks with vent windows '51 through '55.1 came with a different mirror arm to prevent your current problem.
Not true. 47-55.1 use the same arm. See attached screenshot from the parts manual (if it works.)