06A-Month and week of vehicle assembly
in-line-Position of the u-joint flanges on each end are in line.
The most important question to be answered in any of this discussion (at least for 1969 IMO) is why the vast majority of driveshafts had offset u-joints and yet some of the more powerful engine / transmission options had the u-joints in phase.
From a pureley mechanical engineering perspective, in phase u-joints are correct.
In phase u-joints will theoretically maintain the same anglular relationship with respect to the horizontal at both ends of the driveshaft. If everything else works as designed, maintaining this constant angualr relationship will negate the constantaly changing angular velocity at one end of the driveshafti vs. the opposite end.
Out of phase u-joints, by definition, will not maintaitn the same angular relationship at both ends of the driveshaft......why GM / Chevy did this is one of the great mysteries of life.