Getting a new "U" joint has become an issue lately. I might have misunderstood the description so if I have then disregard what follows. The "yoke" of the U-joint is bolted to the output shaft on the transmission. It slides on the drive shaft at least forward and aft. This allows for the difference in drive shaft length as the rear wheels go up and down. The "U" joint should have some but not a lot of movement along the bushings that are trapped with the rather small bolts that secure it to the yoke. Most movement is controlled by the bushing in the torque tube. If you have some signs that the "U" joint is hitting inside the ball then look at the bushing in the torque tube. You may need to replace it and the repair part is called an okie bushing. I hope this is of some help. The gasket set for the transmission is very inexpensive and available from most vendors. So buy the set for the gasket you need.
This is just one example of the gasket set.This is for the 4 speed.
http://www.classicparts.com/1948-67-Transmission-Gasket-Set-4-Speed/productinfo/91-925/#.U0H0iPldW6E

And this is for the 3 speed
http://www.classicparts.com/1947-54-Transmission-Gasket-Set-3-Speed/productinfo/91-905/#.U0H1DvldW6E

Steve H