http://chevy.oldcarmanualproject.com/chevyowner/53tom76.htm

There is a filler plug/hole on the upper passenger side of the SM420 transmission.

The instructions above say to fill to the plug hole.
Other instructions might say: when cold/cool, fill a little below the plug hole (the oil expands when warm).

As posted above, the enclosed drive-shaft transmission u-joint is filled from the transmission.

Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.