shooper,
The rear end casting code might help to identify the rear end. If it starts with an N it is a '55 1st series (the '54 enclosed-driveshaft rear-end started with an M).
If it starts with another letter (other than M or N), it is another year. The second letter indicates the manufacturing location.
By the way, the 1954 1/2 ton truck's heavy duty 3-speed transmission is the same length as the regular 3-speed (and 4-speed) transmissions and they all were used with an enclosed driveshaft (torque tube).
Tim
