Attempted to install GMU-11 bracket; discovered that riveting the top aft skin pulled the upper longerons in slightly; bracket no longer fit. Had to remove GMU-11, grind down bracket ends, elongate bolt holes, deburr, repaint bracket and reinstall GMU-11. Despite mods, reinstalling bracket was difficult with tight access. Dropped washers and tools required removing and reinstalling backboards several times. When GMU-11 bracked was finally installed, removed/modified backboards, installed ELT holder tray and connected transponder antenna cable to antenna.
