Carefully measured fuselage for its width at station 55-13/16, which is as best as I read the plans, the aft face of the vertical web of the seat back support cross member. My measurement came out 1/32” wider than Van’s. I laid out the dimensions for the cabin frame on a 2’ x 4’ piece of melamine-coated MDF, and clamped angles at the end edges of the aft F-631A channels and clamped them firmly in place. Drilled the aft F-631E plate to the inboard ends of the channels.
Drew centerlines on both flanges of both aft channels. Marked hole locations on the outer flanges of both channels. I couldn’t figure out a way to hold my rivet fan in place, so I just started at the bottom of each channel and marked 1.5 inches, until the last space at the top, which came out to 1-1/8, and 5/16 edge distance in the last hole. (This spacing looks good enough I didn’t think it was worthwhile remeasuring for less than 1/64” hole spacing change.)