You will have to change the u-joint as well. Actually half of it needs to be changed when you go to the 4 speed. They are on eBay fairly often in NOS condition. I have had an NOS 3 speed and an NOS 4 speed in my '45 1/2 ton truck. Here is my advice to you. Stick with the 3 speed. Here's why. The GM 4 speed trans of the era is not synchronized and that can be a problem with braking if you miss a downshift. You'll be playing the 3 pedal 2-step in a panic type brake situation. (I don't care how you drive, they will come up when you least expect them)
The 3 speed is synchronized in 2 and 3rd gears, so a nice 3 to 2 downshift with brakes on really helps slow the vehicle down quickly. No need to blip the accelerator to get back to second because of non-sychronization.
I also have the 3.55 gears and they are well worth the expense and effort. Additionally, I have a Truckstell torque tube Overdrive which you cut the torque tube for installation. They are not common, so don't get your hopes up but keep an eye out for guys scrapping original drivelines when they destroy, uh "rod" their vehicle.
Stick with the 3 speed and get the 3.55's , you will be quite happy. The 3 speed is rebuildable and there should be a few around for parts.
