I just did this recently modifying the plastic bezel wasn't too bad. But, enlarging the actual steel dash behind it and fabricating a support bracket was tough, much more labor intensive.
The trim ring on the head unit is the limiting dimension - it just barely fits in the recessed area. You need to get the lower right hand corner as tight to the bezel border as possible. When you get it positioned scribe the inside of the opening. You only want the opening to be as big as the actual head unit - the sleeve goes behind the bezel and gets bracketed to the dash frame.
On the bezel, I drilled the 4 corners and used a jig saw with a metal cutting blade to make the rough opening. Dial it in with a
lot of filing.
The steel dash opening needs to be enlarged as well. I used an air powered high speed cut-off tool. Don't completely detach the pieces you cut out. Leave the bottom connected and bend it back in towards the fire wall - it helps support the head unit sleeve.
Here are some images of how close the fit is:
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