[Shop-talk] retapping a hole

[john]

I have a mirror mount on my truck door that held a rear view mirror.  
Unfortunately the mirror decided it didn't want to stay with my truck.
So, I'm 
looking for a replacement. The mount is drilled and tapped for 
1/4"-28 threads and most of what I find is 1/4-20.  I'm wondering if I can
re-tap the metal over existing threads with a 1/4-20 thread and it will hold...or will I just make a mess of it and make it broke. The drill side for the
two thread sizes are within a 64th or so of being the same and the mount is 7/16" 
or so wide so I have some meat to play with...
 
suggestions?
 
john
 
 
 
another option... drill completely thru mount and find a mirror with a stud 
long enough to pass thru completely
and secure with an acorn nut (or something)  at the back side of the mount
 

 
another caveat... I don't want to remove the mounts from the door... involves removing windows and regulators
to get to the internal nuts.
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