Glad that you know what you have. I brought it up because sometimes people who sell things can be misleading.
I have the same transmission. A previous owner of my truck removed the 3 speed and installed a 4 speed, keeping the torque tube. I found and installed the 1967 4 speed, ditching the torque tube and rear end; I wanted highway gears.
Thoughts:
Don't forget that installing the 4 speed means you will have to either find a new floor plate for the cab or chop a hole in the old flat floor plate.
Gearing wise, installing the 4 speed isn't really an improvement over the 3 speed. It's probably less work to simply fix what's wrong with your 3 speed.
If I was doing it all over again today, I would install a T5 instead of the sm420.
