Marked (with a rivet spacer fan) and drilled holes in seat pan support panels / support brackets prior to install. Drilled out all holes to final size and then deburred all holes. Drilled extra attach holes in both vertical support channels on the rear fuselage because lower left / lower right holes have AN nuts so close behind the hole that rivet installation is not possible from outside, and inside the channel the rivet gun head will not fit. The extra hole is in middle of bottom row and make up for the missing outside lower rivet.