Burb doors are apart, need mirror install help please

[mt Al]

My suburban only has a tiny peep rear view mirror, the wife needs more mirror real estate so we've got some 70 - 72 deluxe rear view mirrors to install from GMC Pauls.

Our '72 pickup's got the same mirrors, so I can measure placement very easy. The mirrors came with backing plates.

Do I just mark the hole locations, smack them with a punch, drill holes and put the backing plate inside the door and screw them in? Stupid question, just want to make sure before I drill.

Some of the undercoating has peeled off of the inside of the door and left quite a bit of surface rust. Should I POR-15 it and re-cover with undercoating?

Any suggestions would be appreciated. Thanks.
(Signature now has geographic location, per recent post request)

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'72 GMC 1/2T 4x4 1500 Super Custom pickup and
'72 GMC 3/4T 4x4 2500 Super Custom suburban.(current)
past rides were: '70 Chev 2wd farm truck
'71 GMC 2wd 1/2T 402 nice!
'72 Chev 2wd 1/2T 396

Bozeman, Montana