Brian - One other essential set of component that can get overlooked when swapping a torque tube 3 speed with a torque tube 4 speed is the front yokes of the u-joint assembly that connects the transmission output shafts to the drive shaft inside the ball collar.
The 3 and 4 speed yoke sets all use the same u-joint, but the forward yokes are actually different, having different casting and part numbers. Below is an extract from a Chevy Master Parts Book with the two different front yokes marked for 1948-54 AD 1/2 tons w/3 and 4 speeds. The rear yoke of the assembly in both cases is the same. IIRC, there is also a difference in the speedometer driven/drive gear sets from 3 and 4 speeds that coincides with the differnt front yokes.
I used the 1/2 ton as a starting point, since you didn't indicate what year and model AD you are working on. Adding those details, either in your post or in your signature line, would help us help you by pointing you in the correct direction for the best answers.
PS - If you're hunting for an SM420, feel free to post an add in our
Truck Parts Wanted forum as it's a great place to get help finding what you need.
