I just swapped out my sm420 for a 3 speed. Just wanted to put my truck in original configuration. (the 4 spd was installed later in its life). I wanted to go back to the 3 on the tree. It is an easy swap. Like others have mentioned the u-joint/yoke is different. The mount for the pre 54 is used for either transmission. The torque tube end is the same as well.
The 1st gear on the sm420 is def not gonna be used unless in a slow parade or working on the farm. My sm420 was pretty noisy, I was shocked how much quieter my truack is now with the 3 speed.
The sm420 is HEAVY so be careful installing it, the 3 speed is LIGHT.
Good luck!
