Tweaked, positioned and drilled the upper and lower rear spar doublers. Clamped the shim in place and drilled four holes in preparation for attachment placement. Unfortunately, I had it placed on the wrong side of the rear spar. Of course, when I flipped the unit over the shim holes were not aligned. I thought I was through for the night and cleaned up my work area. Just the act of clearing the clutter helped my attitude, so I cut and deburred a new shim. After mounting it on the correct side this time, I was ready to install the attachment. Positioning is very important, not only for alignment with future parts, but also for margin around all rivet holes. After numerous fine adjustments, I turned the part over and drilled the 25 holes through the shim and attachment. I'm quite pleased with the outcome of this sub-assembly.