Greg's reply is very helpful.

The use of two mounting-studs (called "guide pins" in the Shop Manual) makes removal/replacement much safer/easier.

Here is information about the SM420 transmission.
ID/casting codes are listed.

The rear housing for the torque-tube u-joint can be swapped on/off any year SM420.

u-joint parts are available from several vendors

The Shop Manual describes removal/installation:
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0713.htm