I put screws and nutplates on the two pieces to attach the two parts of the fuel selector mount together. Since the longeron is 1/8 thick I simply attached it under the other part of the bracket which made the selector bracket flush with the longeron.
The inboard bracket is only 1/16 thick so I set the bracket in my milling machine and trimmed 1/16 off of the thickness of the mount plate so now the inboard side is also flush.
Because the mount sits at a slight angle from right to left because of the height of the two attach points is slightly different I put a slight bevel where the bracket is joined and I put a slight bend in the bracket just to the left of the screws in the lap joint to allow the bracket to sit flush on the longeron. The longeron has enough twist also that the bend allowed the bracket to sit flat on the bottom of the longeron for riveting purposes.
I marked, drilled, and countersunk the holes for the rivets.