This is the upper left hand firewall bracket (WD-1002). View looking up.
The bracket is made from 0.063" 4130 steel.
Here is a closer view of the holes. View looking forward.
As you can see the edge distance reduces to almost nothing in the aft holes.
Note a total of 9 rivets in the upper tab of the bracket.
Picture of the WD-1002 upper flange after the cut. I used other pieces of steel to shield the existing parts while cutting to prevent any accidental strikes from the cutting wheel.