Wow, I just read this whole thread and would be confused as to what to do. I just finished rebuilding my SM-318 3-speed on my ‘52 and can tell you that there is no extra fill for the u-joint , the tranny migrates lube to the u-joint.
In reference to the four speed SM-420, the link to the ‘52 Operators Manual tells all. I would however, fill the u-joint cavity after a rebuild, but after that, the transmission oil will migrate to keep it lubricated.
http://chevy.oldcarmanualproject.com/chevyowner/52tom98.htm
