As mentioned previously, the vertical bracket which goes on the inboard left stiffener needed shortened to clear the rudder pedal support.

Since the flange of the brackets which will be attached to the firewall stiffeners is 1/2 inch wide, I originally marked the holes at 1/4 inch from the edge of the firewall stiffener.

Afterwards, I decided to move those holes back an extra 1/16 inch. I figured that from a structural standpoint, it would be best to have the holes closer to the other flange of the stiffener (which is riveted to the firewall). The sub panel brackets are not at all structural, so as long as they stay attached, edge distance isn't important.

I didn't realize it when I drilled the top right bracket, but the corner of the bracket was interfering with the right inboard vertical firewall stiffener (which was joggled).

...so I filed some relief. The holes were drilled 3/32 originally which allows me room to adjust the position before final drilling.

I ran into a similar situation when fitting the vertical bracket to the inboard vertical stiffeners.

To drill the holes attaching the vertical brackets to the inboard vertical stiffeners, I moved my plywood flooring from the baggage bay to the forward fuselage. This allowed me to actually climb in the forward fuselage to get better access with the angle drill.

I made up a drilling guide so the spacing would be uniform...

...making it easy to drill the holes in the correct locations (both fore/aft and up/down).

After several hours, I had the top and inboard brackets drilled to the firewall structure.