It's not as bad as it seems. The League was different back then. Lots more down field passing = horrible completion percentage. Heres how Archie stacks up against a few other from the "early" years:
the number on the left is where they rank all time:
rank rating
93 Ron Jaworski 72.78
106 Terry Bradshaw 70.92
129 Joe Namath 65.46
123 Jim Plunkett 67.5