Do you have an Operator's Manual and a Shop Manual? They are on-line (see links below).

http://chevy.oldcarmanualproject.com/chevyowner/52tom77.htm

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0005.htm

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0730.htm

6 pints of 90 weight gear oil in transmission (same oil is used in the rear end).

The rear end is filled through a plug in the rear cover.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0410.htm

The transmission fluid also lubricates the enclosed u-joint (see last link above). If you empty the u-joint housing, you should put about a 1/2 pint of oil in the plug-hole near the rear of the transmission case.

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.