All joking aside (or would that be "yokeing?), the part in question is called an output yoke. From what you are describing you are changing from an SM420 to a later style saginaw 4 speed.
You are probably going to need a new drive shaft to do this. Just tell them at the drive shaft shop what trans you are using, they will put the correct yoke on it. If you only need a yoke, try Speedway/Summit/Jegs. They have them also.
