I sourced the correct T3 lower mount brackets new from classic parts via Brickwerks (251-857-541) so I never modified the LT lower brackets or drilled a new hole in the door. Some others have.
First of all the mirrors I have are heated (I'm going to wire them in later)
You can see there is a nice slot between the lower mount holes for the wire to feed into

I drilled a 6mm hole in the lower mount gasket for the wire to go through

The top mount on LT arms doesn't quite line up with the angled part of the cab door but the arms are fairly flexible and it can be enticed to fit. Rather than line this bracket up when screwing it in, which can strip the screw hole, I bolted the mirror to the door using the lower mount and marked the hole positions of the top mount on the door.
I then removed the top mount from the arm (watch you don't lose the little spacer bushes) and using the top marked hole position as a guide I re-positioned the top bracket to sit flush to the angled part of the cab door, re-marked the holes, drilled and fitted the top mount

I then refitted the mirror arm to the top bracket. You can see that it does not sit straight

This is not a problem because you can pull the arm straight as you fit the two screws in the bottom bracket.
Here’s a comparison between the standard mirrors and the LT ones

And here’s the job done
