After thinking about potential causes for the twist in the rudder, and spending several hours checking component dimensions, bench leveling, and leveling of of the two fixturing blocks, I determined that the 0.1 degree resolution of my digital level was insufficient. So I changed to a good, standard bubble level, and releveled the two fixturing blocks.
Remounted the rudder on the fixture and updrilled the rivet holes with the rudder clamped to the correctly leveled fixturing blocks. The updrilling corrected the minor hole mislocation caused by the misaligned fixturing.
