The 54-59 "pumpkin" or center section is a direct bolt on change. The challenge is that you would have to have new spring perches welded to your axle. If you used the entire 54-59 to 59 axle, you will likely have to grind off the existing spring perches and have them welded on for proper spring spacing and pinion angle.
To give you more background... a torque tube is great for delivering torque. The U-joints are held at the optimal angle for transmitting torque by the tube. The tube prevents the axle from rolling under acceleration.
On an open driveline, there is no torque tube to prevent the axle from rolling due to torque from the wheels during acceleration. The anti role is done by having a spring perch welded to the axle. The perch is flat on the spring side, and depending on the application can sit under the spring, or over the spring. The axle would be bolted to the spring with long u bolts, with the perches between the springs, axles and u bolts
Pinion angle also has to be set properly with an open driveline. There is little room for error as the efficiency of transmitting torque is proportional to the angle of the pinion. The angle also changes under driving conditions and load. One can not expect to take an axle from one vehicle to another and have those pinion angles match, as there are variables such as length of driveline, ride height, weight, etc.
To answer your above question simply... any rear swap in an AD truck from torque tube to open driveline will not be a direct bolt in. It will take proper setup, measuring and welding. once the axle has been setup to the application, and the perches have been welded to the axle, it will simply bolt in place. The above statement is also true if you just change out the pumpkin.
