Mirror holes, new ones to drill others to fill

[hotwls13]

I'm replacing the big ugly mirrors with the single post, 3 hole, rectangular mirrors.

Existing top holes are fine (I need to add the 3rd hole to each side).

My questions are:

1. What did u guys use as nuts (I bought the stainless bolts with my mirrors, but the upper holes are just empty)? I'm assuming just use stainless flat+lock washers and appropriate nut. Or should I use stainless flat washer with nylock nuts?

2. I was planning to put cutoff bolts in the lower holes then use bondo glass as top filler. But, the embedded nuts appear to be pertruding outside the door surface. Do I grind them down or bang them in so they either fall out or are inward??